[PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity

[PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity

[Gabriele Monaco]

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

When a process reduces its number of threads or clears bits in its CPU
affinity mask, the mm_cid allocation should eventually converge towards
smaller values.

However, the change introduced by:

commit 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency
IDs for intermittent workloads")

adds a per-mm/CPU recent_cid which is never unset unless a thread
migrates.

This is a tradeoff between:

A) Preserving cache locality after a transition from many threads to few
   threads, or after reducing the hamming weight of the allowed CPU mask.

B) Making the mm_cid upper bounds wrt nr threads and allowed CPU mask
   easy to document and understand.

C) Allowing applications to eventually react to mm_cid compaction after
   reduction of the nr threads or allowed CPU mask, making the tracking
   of mm_cid compaction easier by shrinking it back towards 0 or not.

D) Making sure applications that periodically reduce and then increase
   again the nr threads or allowed CPU mask still benefit from good
   cache locality with mm_cid.

Introduce the following changes:

* After shrinking the number of threads or reducing the number of
  allowed CPUs, reduce the value of max_nr_cid so expansion of CID
  allocation will preserve cache locality if the number of threads or
  allowed CPUs increase again.

* Only re-use a recent_cid if it is within the max_nr_cid upper bound,
  else find the first available CID.

Fixes: 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads")
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Marco Elver <elver@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Tested-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/mm_types.h |  7 ++++---
 kernel/sched/sched.h     | 25 ++++++++++++++++++++++---
 2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 6b27db7f94963..0234f14f2aa6b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -875,10 +875,11 @@ struct mm_struct {
 		 */
 		unsigned int nr_cpus_allowed;
 		/**
-		 * @max_nr_cid: Maximum number of concurrency IDs allocated.
+		 * @max_nr_cid: Maximum number of allowed concurrency
+		 *              IDs allocated.
 		 *
-		 * Track the highest number of concurrency IDs allocated for the
-		 * mm.
+		 * Track the highest number of allowed concurrency IDs
+		 * allocated for the mm.
 		 */
 		atomic_t max_nr_cid;
 		/**
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 38e0e323dda26..606c96b74ebfa 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3698,10 +3698,28 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm)
 {
 	struct cpumask *cidmask = mm_cidmask(mm);
 	struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid;
-	int cid = __this_cpu_read(pcpu_cid->recent_cid);
+	int cid, max_nr_cid, allowed_max_nr_cid;
 
+	/*
+	 * After shrinking the number of threads or reducing the number
+	 * of allowed cpus, reduce the value of max_nr_cid so expansion
+	 * of cid allocation will preserve cache locality if the number
+	 * of threads or allowed cpus increase again.
+	 */
+	max_nr_cid = atomic_read(&mm->max_nr_cid);
+	while ((allowed_max_nr_cid = min_t(int, READ_ONCE(mm->nr_cpus_allowed),
+					   atomic_read(&mm->mm_users))),
+	       max_nr_cid > allowed_max_nr_cid) {
+		/* atomic_try_cmpxchg loads previous mm->max_nr_cid into max_nr_cid. */
+		if (atomic_try_cmpxchg(&mm->max_nr_cid, &max_nr_cid, allowed_max_nr_cid)) {
+			max_nr_cid = allowed_max_nr_cid;
+			break;
+		}
+	}
 	/* Try to re-use recent cid. This improves cache locality. */
-	if (!mm_cid_is_unset(cid) && !cpumask_test_and_set_cpu(cid, cidmask))
+	cid = __this_cpu_read(pcpu_cid->recent_cid);
+	if (!mm_cid_is_unset(cid) && cid < max_nr_cid &&
+	    !cpumask_test_and_set_cpu(cid, cidmask))
 		return cid;
 	/*
 	 * Expand cid allocation if the maximum number of concurrency
@@ -3709,8 +3727,9 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm)
 	 * and number of threads. Expanding cid allocation as much as
 	 * possible improves cache locality.
 	 */
-	cid = atomic_read(&mm->max_nr_cid);
+	cid = max_nr_cid;
 	while (cid < READ_ONCE(mm->nr_cpus_allowed) && cid < atomic_read(&mm->mm_users)) {
+		/* atomic_try_cmpxchg loads previous mm->max_nr_cid into cid. */
 		if (!atomic_try_cmpxchg(&mm->max_nr_cid, &cid, cid + 1))
 			continue;
 		if (!cpumask_test_and_set_cpu(cid, cidmask))
-- 
2.48.1

[PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Gabriele Monaco]

Currently, the task_mm_cid_work function is called in a task work
triggered by a scheduler tick to frequently compact the mm_cids of each
process. This can delay the execution of the corresponding thread for
the entire duration of the function, negatively affecting the response
in case of real time tasks. In practice, we observe task_mm_cid_work
increasing the latency of 30-35us on a 128 cores system, this order of
magnitude is meaningful under PREEMPT_RT.

Run the task_mm_cid_work in a new delayed work connected to the
mm_struct rather than in the task context before returning to
userspace.

This delayed work is initialised while allocating the mm and disabled
before freeing it, its execution is no longer triggered by scheduler
ticks but run periodically based on the defined MM_CID_SCAN_DELAY.

The main advantage of this change is that the function can be offloaded
to a different CPU and even preempted by RT tasks.

Moreover, this new behaviour could be more predictable with periodic
tasks with short runtime, which may rarely run during a scheduler tick.
Now, the work is always scheduled with the same periodicity for each mm
(though the periodicity is not guaranteed due to interference from other
tasks, but mm_cid compaction is mostly best effort).

To avoid excessively increased runtime, we quickly return from the
function if we have no work to be done (i.e. no mm_cid is allocated).
This is helpful for tasks that sleep for a long time, but also for
terminated task. We are no longer following the process' state, hence
the function continues to run after a process terminates but before its
mm is freed.

Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/mm_types.h | 16 ++++++----
 include/linux/sched.h    |  1 -
 kernel/sched/core.c      | 66 +++++-----------------------------------
 kernel/sched/sched.h     |  7 -----
 4 files changed, 18 insertions(+), 72 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0234f14f2aa6b..3aeadb519cac5 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -861,12 +861,6 @@ struct mm_struct {
 		 * runqueue locks.
 		 */
 		struct mm_cid __percpu *pcpu_cid;
-		/*
-		 * @mm_cid_next_scan: Next mm_cid scan (in jiffies).
-		 *
-		 * When the next mm_cid scan is due (in jiffies).
-		 */
-		unsigned long mm_cid_next_scan;
 		/**
 		 * @nr_cpus_allowed: Number of CPUs allowed for mm.
 		 *
@@ -889,6 +883,7 @@ struct mm_struct {
 		 * mm nr_cpus_allowed updates.
 		 */
 		raw_spinlock_t cpus_allowed_lock;
+		struct delayed_work mm_cid_work;
 #endif
 #ifdef CONFIG_MMU
 		atomic_long_t pgtables_bytes;	/* size of all page tables */
@@ -1180,11 +1175,16 @@ static inline void vma_iter_init(struct vma_iterator *vmi,
 
 #ifdef CONFIG_SCHED_MM_CID
 
+#define SCHED_MM_CID_PERIOD_NS	(100ULL * 1000000)	/* 100ms */
+#define MM_CID_SCAN_DELAY	100			/* 100ms */
+
 enum mm_cid_state {
 	MM_CID_UNSET = -1U,		/* Unset state has lazy_put flag set. */
 	MM_CID_LAZY_PUT = (1U << 31),
 };
 
+extern void task_mm_cid_work(struct work_struct *work);
+
 static inline bool mm_cid_is_unset(int cid)
 {
 	return cid == MM_CID_UNSET;
@@ -1257,12 +1257,16 @@ static inline int mm_alloc_cid_noprof(struct mm_struct *mm, struct task_struct *
 	if (!mm->pcpu_cid)
 		return -ENOMEM;
 	mm_init_cid(mm, p);
+	INIT_DELAYED_WORK(&mm->mm_cid_work, task_mm_cid_work);
+	schedule_delayed_work(&mm->mm_cid_work,
+			      msecs_to_jiffies(MM_CID_SCAN_DELAY));
 	return 0;
 }
 #define mm_alloc_cid(...)	alloc_hooks(mm_alloc_cid_noprof(__VA_ARGS__))
 
 static inline void mm_destroy_cid(struct mm_struct *mm)
 {
+	disable_delayed_work_sync(&mm->mm_cid_work);
 	free_percpu(mm->pcpu_cid);
 	mm->pcpu_cid = NULL;
 }
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9632e3318e0d6..515b15f946cac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1397,7 +1397,6 @@ struct task_struct {
 	int				last_mm_cid;	/* Most recent cid in mm */
 	int				migrate_from_cpu;
 	int				mm_cid_active;	/* Whether cid bitmap is active */
-	struct callback_head		cid_work;
 #endif
 
 	struct tlbflush_unmap_batch	tlb_ubc;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 165c90ba64ea9..c65003ab8c55b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4524,7 +4524,6 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->wake_entry.u_flags = CSD_TYPE_TTWU;
 	p->migration_pending = NULL;
 #endif
-	init_sched_mm_cid(p);
 }
 
 DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
@@ -5662,7 +5661,6 @@ void sched_tick(void)
 		resched_latency = cpu_resched_latency(rq);
 	calc_global_load_tick(rq);
 	sched_core_tick(rq);
-	task_tick_mm_cid(rq, donor);
 	scx_tick(rq);
 
 	rq_unlock(rq, &rf);
@@ -10528,38 +10526,17 @@ static void sched_mm_cid_remote_clear_weight(struct mm_struct *mm, int cpu,
 	sched_mm_cid_remote_clear(mm, pcpu_cid, cpu);
 }
 
-static void task_mm_cid_work(struct callback_head *work)
+void task_mm_cid_work(struct work_struct *work)
 {
-	unsigned long now = jiffies, old_scan, next_scan;
-	struct task_struct *t = current;
 	struct cpumask *cidmask;
-	struct mm_struct *mm;
+	struct delayed_work *delayed_work = container_of(work, struct delayed_work, work);
+	struct mm_struct *mm = container_of(delayed_work, struct mm_struct, mm_cid_work);
 	int weight, cpu;
 
-	SCHED_WARN_ON(t != container_of(work, struct task_struct, cid_work));
-
-	work->next = work;	/* Prevent double-add */
-	if (t->flags & PF_EXITING)
-		return;
-	mm = t->mm;
-	if (!mm)
-		return;
-	old_scan = READ_ONCE(mm->mm_cid_next_scan);
-	next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY);
-	if (!old_scan) {
-		unsigned long res;
-
-		res = cmpxchg(&mm->mm_cid_next_scan, old_scan, next_scan);
-		if (res != old_scan)
-			old_scan = res;
-		else
-			old_scan = next_scan;
-	}
-	if (time_before(now, old_scan))
-		return;
-	if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan, next_scan))
-		return;
 	cidmask = mm_cidmask(mm);
+	/* Nothing to clear for now */
+	if (cpumask_empty(cidmask))
+		goto out;
 	/* Clear cids that were not recently used. */
 	for_each_possible_cpu(cpu)
 		sched_mm_cid_remote_clear_old(mm, cpu);
@@ -10570,35 +10547,8 @@ static void task_mm_cid_work(struct callback_head *work)
 	 */
 	for_each_possible_cpu(cpu)
 		sched_mm_cid_remote_clear_weight(mm, cpu, weight);
-}
-
-void init_sched_mm_cid(struct task_struct *t)
-{
-	struct mm_struct *mm = t->mm;
-	int mm_users = 0;
-
-	if (mm) {
-		mm_users = atomic_read(&mm->mm_users);
-		if (mm_users == 1)
-			mm->mm_cid_next_scan = jiffies + msecs_to_jiffies(MM_CID_SCAN_DELAY);
-	}
-	t->cid_work.next = &t->cid_work;	/* Protect against double add */
-	init_task_work(&t->cid_work, task_mm_cid_work);
-}
-
-void task_tick_mm_cid(struct rq *rq, struct task_struct *curr)
-{
-	struct callback_head *work = &curr->cid_work;
-	unsigned long now = jiffies;
-
-	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) ||
-	    work->next != work)
-		return;
-	if (time_before(now, READ_ONCE(curr->mm->mm_cid_next_scan)))
-		return;
-
-	/* No page allocation under rq lock */
-	task_work_add(curr, work, TWA_RESUME);
+out:
+	schedule_delayed_work(delayed_work, msecs_to_jiffies(MM_CID_SCAN_DELAY));
 }
 
 void sched_mm_cid_exit_signals(struct task_struct *t)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 606c96b74ebfa..fc613d9090bed 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3622,16 +3622,11 @@ extern void sched_dynamic_update(int mode);
 
 #ifdef CONFIG_SCHED_MM_CID
 
-#define SCHED_MM_CID_PERIOD_NS	(100ULL * 1000000)	/* 100ms */
-#define MM_CID_SCAN_DELAY	100			/* 100ms */
-
 extern raw_spinlock_t cid_lock;
 extern int use_cid_lock;
 
 extern void sched_mm_cid_migrate_from(struct task_struct *t);
 extern void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t);
-extern void task_tick_mm_cid(struct rq *rq, struct task_struct *curr);
-extern void init_sched_mm_cid(struct task_struct *t);
 
 static inline void __mm_cid_put(struct mm_struct *mm, int cid)
 {
@@ -3899,8 +3894,6 @@ static inline void switch_mm_cid(struct rq *rq,
 static inline void switch_mm_cid(struct rq *rq, struct task_struct *prev, struct task_struct *next) { }
 static inline void sched_mm_cid_migrate_from(struct task_struct *t) { }
 static inline void sched_mm_cid_migrate_to(struct rq *dst_rq, struct task_struct *t) { }
-static inline void task_tick_mm_cid(struct rq *rq, struct task_struct *curr) { }
-static inline void init_sched_mm_cid(struct task_struct *t) { }
 #endif /* !CONFIG_SCHED_MM_CID */
 
 extern u64 avg_vruntime(struct cfs_rq *cfs_rq);
-- 
2.48.1

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[kernel test robot]

Hello,

kernel test robot noticed "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:

commit: 287adf9e9c1fa8a0e2b50ab1a1de3e4572a8ccd2 ("[PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work")
url: https://github.com/intel-lab-lkp/linux/commits/Gabriele-Monaco/sched-Compact-RSEQ-concurrency-IDs-with-reduced-threads-and-affinity/20250210-233547
patch link: https://lore.kernel.org/all/20250210153253.460471-3-gmonaco@redhat.com/
patch subject: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

in testcase: boot

config: x86_64-randconfig-004-20250211
compiler: clang-19
test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G

(please refer to attached dmesg/kmsg for entire log/backtrace)



If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <oliver.sang@intel.com>
| Closes: https://lore.kernel.org/oe-lkp/202502131405.1ba0803f-lkp@intel.com


[    2.640924][    T0] ------------[ cut here ]------------
[ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495 __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[    2.642874][    T0] Modules linked in:
[    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.14.0-rc2-00002-g287adf9e9c1f #1
[    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[ 2.645943][ T0] RIP: 0010:__queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[ 2.646755][ T0] Code: 44 89 fe 4c 89 33 e8 6d 24 19 00 48 83 c4 08 5b 41 5c 41 5d 41 5e 41 5f 5d 31 c0 31 c9 31 ff 31 d2 31 f6 c3 e8 7f 87 25 00 90 <0f> 0b 90 e9 58 fd ff ff e8 71 87 25 00 90 0f 0b 90 e9 80 fd ff ff
All code
========
   0:	44 89 fe             	mov    %r15d,%esi
   3:	4c 89 33             	mov    %r14,(%rbx)
   6:	e8 6d 24 19 00       	call   0x192478
   b:	48 83 c4 08          	add    $0x8,%rsp
   f:	5b                   	pop    %rbx
  10:	41 5c                	pop    %r12
  12:	41 5d                	pop    %r13
  14:	41 5e                	pop    %r14
  16:	41 5f                	pop    %r15
  18:	5d                   	pop    %rbp
  19:	31 c0                	xor    %eax,%eax
  1b:	31 c9                	xor    %ecx,%ecx
  1d:	31 ff                	xor    %edi,%edi
  1f:	31 d2                	xor    %edx,%edx
  21:	31 f6                	xor    %esi,%esi
  23:	c3                   	ret
  24:	e8 7f 87 25 00       	call   0x2587a8
  29:	90                   	nop
  2a:*	0f 0b                	ud2		<-- trapping instruction
  2c:	90                   	nop
  2d:	e9 58 fd ff ff       	jmp    0xfffffffffffffd8a
  32:	e8 71 87 25 00       	call   0x2587a8
  37:	90                   	nop
  38:	0f 0b                	ud2
  3a:	90                   	nop
  3b:	e9 80 fd ff ff       	jmp    0xfffffffffffffdc0

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	90                   	nop
   3:	e9 58 fd ff ff       	jmp    0xfffffffffffffd60
   8:	e8 71 87 25 00       	call   0x25877e
   d:	90                   	nop
   e:	0f 0b                	ud2
  10:	90                   	nop
  11:	e9 80 fd ff ff       	jmp    0xfffffffffffffd96
[    2.649301][    T0] RSP: 0000:ffffffffab007d80 EFLAGS: 00010046
[    2.650081][    T0] RAX: 0000000000000000 RBX: ffff888100090b98 RCX: 0000000000000000
[    2.651084][    T0] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[    2.652078][    T0] RBP: ffffffffab007db0 R08: 0000000000000000 R09: 0000000000000000
[    2.653077][    T0] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[    2.654101][    T0] R13: 0000000000000040 R14: 0000000000000019 R15: 0000000000000040
[    2.655112][    T0] FS:  0000000000000000(0000) GS:ffff8883af200000(0000) knlGS:0000000000000000
[    2.656263][    T0] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    2.657103][    T0] CR2: ffff88843ffff000 CR3: 00000001d3617000 CR4: 00000000000000b0
[    2.658128][    T0] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    2.659147][    T0] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    2.660143][    T0] Call Trace:
[    2.660553][    T0]  <TASK>
[ 2.660934][ T0] ? show_regs (arch/x86/kernel/dumpstack.c:479) 
[ 2.661524][ T0] ? __warn (kernel/panic.c:748) 
[ 2.662065][ T0] ? __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[ 2.662796][ T0] ? __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[ 2.663535][ T0] ? report_bug (lib/bug.c:?) 
[ 2.664119][ T0] ? __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[ 2.664836][ T0] ? handle_bug (arch/x86/kernel/traps.c:285) 
[ 2.665404][ T0] ? exc_invalid_op (arch/x86/kernel/traps.c:309 (discriminator 1)) 
[ 2.665848][ T0] ? asm_exc_invalid_op (arch/x86/include/asm/idtentry.h:621) 
[ 2.666292][ T0] ? __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
[ 2.666762][ T0] queue_delayed_work_on (kernel/workqueue.c:?) 
[ 2.667215][ T0] mm_init (include/linux/workqueue.h:? include/linux/workqueue.h:817 include/linux/mm_types.h:1261 kernel/fork.c:1310) 
[ 2.667575][ T0] mm_alloc (kernel/fork.c:?) 
[ 2.667916][ T0] ? x86_64_start_reservations (usercopy_64.c:?) 
[ 2.668400][ T0] poking_init (arch/x86/mm/init.c:822) 
[ 2.668777][ T0] ? x86_64_start_reservations (usercopy_64.c:?) 
[ 2.669266][ T0] start_kernel (init/main.c:959) 
[ 2.669671][ T0] x86_64_start_reservations (usercopy_64.c:?) 
[ 2.670140][ T0] x86_64_start_kernel (arch/x86/kernel/head64.c:445 (discriminator 2)) 
[ 2.670574][ T0] common_startup_64 (arch/x86/kernel/head_64.S:421) 
[    2.671018][    T0]  </TASK>
[    2.671271][    T0] irq event stamp: 0
[ 2.671595][ T0] hardirqs last enabled at (0): 0x0 
[ 2.672192][ T0] hardirqs last disabled at (0): 0x0 
[ 2.672789][ T0] softirqs last enabled at (0): 0x0 
[ 2.673386][ T0] softirqs last disabled at (0): 0x0 
[    2.674005][    T0] ---[ end trace 0000000000000000 ]---


The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20250213/202502131405.1ba0803f-lkp@intel.com



-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Gabriele Monaco]

On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
> kernel test robot noticed
> "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
> 
> [    2.640924][    T0] ------------[ cut here ]------------
> [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
> __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9)) 
> [    2.642874][    T0] Modules linked in:
> [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
> 6.14.0-rc2-00002-g287adf9e9c1f #1
> [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
> PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
> (kernel/workqueue.c:2495 (discriminator 9)) 

There seem to be major problems with this configuration, I'm trying to
understand what's wrong but, for the time being, this patchset is not
ready for inclusion.

Gabriele

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Mathieu Desnoyers]

On 2025-02-13 08:25, Gabriele Monaco wrote:
> On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
>> kernel test robot noticed
>> "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
>>
>> [    2.640924][    T0] ------------[ cut here ]------------
>> [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
>> __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
>> [    2.642874][    T0] Modules linked in:
>> [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
>> 6.14.0-rc2-00002-g287adf9e9c1f #1
>> [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
>> PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
>> (kernel/workqueue.c:2495 (discriminator 9))
> 
> There seem to be major problems with this configuration, I'm trying to
> understand what's wrong but, for the time being, this patchset is not
> ready for inclusion.

I think there is an issue with the order of init functions at boot.

poking_init() calls mm_alloc(), which ends up calling mm_init().

The WARN_ON() is about a NULL wq pointer, which I suspect happens
if poking_init() is called before workqueue_init_early(), which
allocates system_wq.

Indeed, in start_kernel(), poking_init() is called before
workqueue_init_early().

I'm not sure what are the init order dependencies across subsystems here.
There is the following order in start_kernel():

[...]
         mm_core_init();
         poking_init();
         ftrace_init();

         /* trace_printk can be enabled here */
         early_trace_init();

         /*
          * Set up the scheduler prior starting any interrupts (such as the
          * timer interrupt). Full topology setup happens at smp_init()
          * time - but meanwhile we still have a functioning scheduler.
          */
         sched_init();

         if (WARN(!irqs_disabled(),
                  "Interrupts were enabled *very* early, fixing it\n"))
                 local_irq_disable();
         radix_tree_init();
         maple_tree_init();

         /*
          * Set up housekeeping before setting up workqueues to allow the unbound
          * workqueue to take non-housekeeping into account.
          */
         housekeeping_init();

         /*
          * Allow workqueue creation and work item queueing/cancelling
          * early.  Work item execution depends on kthreads and starts after
          * workqueue_init().
          */
         workqueue_init_early();
[...]

So either we find a way to reorder this, or we make sure poking_init()
does not require the workqueue.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Gabriele Monaco]

On Thu, 2025-02-13 at 08:55 -0500, Mathieu Desnoyers wrote:
> On 2025-02-13 08:25, Gabriele Monaco wrote:
> > On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
> > > kernel test robot noticed
> > > "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
> > > 
> > > [    2.640924][    T0] ------------[ cut here ]------------
> > > [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at
> > > kernel/workqueue.c:2495
> > > __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
> > > [    2.642874][    T0] Modules linked in:
> > > [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not
> > > tainted
> > > 6.14.0-rc2-00002-g287adf9e9c1f #1
> > > [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
> > > PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > > [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
> > > (kernel/workqueue.c:2495 (discriminator 9))
> > 
> > There seem to be major problems with this configuration, I'm trying
> > to
> > understand what's wrong but, for the time being, this patchset is
> > not
> > ready for inclusion.
> 
> I think there is an issue with the order of init functions at boot.
> 
> poking_init() calls mm_alloc(), which ends up calling mm_init().
> 
> The WARN_ON() is about a NULL wq pointer, which I suspect happens
> if poking_init() is called before workqueue_init_early(), which
> allocates system_wq.
> 
> Indeed, in start_kernel(), poking_init() is called before
> workqueue_init_early().
> 
> I'm not sure what are the init order dependencies across subsystems
> here.
> There is the following order in start_kernel():
> 
> [...]
>          mm_core_init();
>          poking_init();
>          ftrace_init();
> 
>          /* trace_printk can be enabled here */
>          early_trace_init();
> 
>          /*
>           * Set up the scheduler prior starting any interrupts (such
> as the
>           * timer interrupt). Full topology setup happens at
> smp_init()
>           * time - but meanwhile we still have a functioning
> scheduler.
>           */
>          sched_init();
> 
>          if (WARN(!irqs_disabled(),
>                   "Interrupts were enabled *very* early, fixing
> it\n"))
>                  local_irq_disable();
>          radix_tree_init();
>          maple_tree_init();
> 
>          /*
>           * Set up housekeeping before setting up workqueues to allow
> the unbound
>           * workqueue to take non-housekeeping into account.
>           */
>          housekeeping_init();
> 
>          /*
>           * Allow workqueue creation and work item
> queueing/cancelling
>           * early.  Work item execution depends on kthreads and
> starts after
>           * workqueue_init().
>           */
>          workqueue_init_early();
> [...]
> 
> So either we find a way to reorder this, or we make sure
> poking_init()
> does not require the workqueue.
> 
> Thanks,
> 
> Mathieu
> 

Nice suggestion! That seems the culprit..

From the full dmesg of the failure I've seen there's also a problem
with disabling the delayed work synchronously, since mmdrop cannot
sleep if we are not in PREEMPT_RT.

I'm trying to come with some satisfactory solution for both, ideally:
1. the delayed work is not needed in early boot, we may have a better
place where to start it
2. we can cancel the work asynchronously on mmdrop and abort it if the
pcpu_cid is null, but it seems racy, perhaps there's a better place for
that too

Thanks,
Gabriele

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Mathieu Desnoyers]

On 2025-02-13 08:25, Gabriele Monaco wrote:
> On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
>> kernel test robot noticed
>> "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
>>
>> [    2.640924][    T0] ------------[ cut here ]------------
>> [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
>> __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
>> [    2.642874][    T0] Modules linked in:
>> [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
>> 6.14.0-rc2-00002-g287adf9e9c1f #1
>> [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
>> PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
>> (kernel/workqueue.c:2495 (discriminator 9))
> 
> There seem to be major problems with this configuration, I'm trying to
> understand what's wrong but, for the time being, this patchset is not
> ready for inclusion.

Patch 1/3 has been ready for a while though. Can we post it separately
for inclusion ?

Thanks,

Mathieu

> 
> Gabriele
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Gabriele Monaco]

On Thu, 2025-02-13 at 09:52 -0500, Mathieu Desnoyers wrote:
> On 2025-02-13 08:25, Gabriele Monaco wrote:
> > On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
> > > kernel test robot noticed
> > > "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
> > > 
> > > [    2.640924][    T0] ------------[ cut here ]------------
> > > [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at
> > > kernel/workqueue.c:2495
> > > __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
> > > [    2.642874][    T0] Modules linked in:
> > > [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not
> > > tainted
> > > 6.14.0-rc2-00002-g287adf9e9c1f #1
> > > [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
> > > PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > > [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
> > > (kernel/workqueue.c:2495 (discriminator 9))
> > 
> > There seem to be major problems with this configuration, I'm trying
> > to
> > understand what's wrong but, for the time being, this patchset is
> > not
> > ready for inclusion.
> 
> Patch 1/3 has been ready for a while though. Can we post it
> separately
> for inclusion ?
> 

I would say so, there's no need to delay it further.

Thanks,
Gabriele

Re: [PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity

[Mathieu Desnoyers]

On 2025-02-10 10:32, Gabriele Monaco wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 

Peter, Ingo, this patch has been ready for inclusion for a while. The
rest of this series does not seem to be quite ready yet, but can we at
least merge this patch into tip ?

Thanks,

Mathieu

> When a process reduces its number of threads or clears bits in its CPU
> affinity mask, the mm_cid allocation should eventually converge towards
> smaller values.
> 
> However, the change introduced by:
> 
> commit 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency
> IDs for intermittent workloads")
> 
> adds a per-mm/CPU recent_cid which is never unset unless a thread
> migrates.
> 
> This is a tradeoff between:
> 
> A) Preserving cache locality after a transition from many threads to few
>     threads, or after reducing the hamming weight of the allowed CPU mask.
> 
> B) Making the mm_cid upper bounds wrt nr threads and allowed CPU mask
>     easy to document and understand.
> 
> C) Allowing applications to eventually react to mm_cid compaction after
>     reduction of the nr threads or allowed CPU mask, making the tracking
>     of mm_cid compaction easier by shrinking it back towards 0 or not.
> 
> D) Making sure applications that periodically reduce and then increase
>     again the nr threads or allowed CPU mask still benefit from good
>     cache locality with mm_cid.
> 
> Introduce the following changes:
> 
> * After shrinking the number of threads or reducing the number of
>    allowed CPUs, reduce the value of max_nr_cid so expansion of CID
>    allocation will preserve cache locality if the number of threads or
>    allowed CPUs increase again.
> 
> * Only re-use a recent_cid if it is within the max_nr_cid upper bound,
>    else find the first available CID.
> 
> Fixes: 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads")
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Marco Elver <elver@google.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Tested-by: Gabriele Monaco <gmonaco@redhat.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   include/linux/mm_types.h |  7 ++++---
>   kernel/sched/sched.h     | 25 ++++++++++++++++++++++---
>   2 files changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6b27db7f94963..0234f14f2aa6b 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -875,10 +875,11 @@ struct mm_struct {
>   		 */
>   		unsigned int nr_cpus_allowed;
>   		/**
> -		 * @max_nr_cid: Maximum number of concurrency IDs allocated.
> +		 * @max_nr_cid: Maximum number of allowed concurrency
> +		 *              IDs allocated.
>   		 *
> -		 * Track the highest number of concurrency IDs allocated for the
> -		 * mm.
> +		 * Track the highest number of allowed concurrency IDs
> +		 * allocated for the mm.
>   		 */
>   		atomic_t max_nr_cid;
>   		/**
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 38e0e323dda26..606c96b74ebfa 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3698,10 +3698,28 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm)
>   {
>   	struct cpumask *cidmask = mm_cidmask(mm);
>   	struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid;
> -	int cid = __this_cpu_read(pcpu_cid->recent_cid);
> +	int cid, max_nr_cid, allowed_max_nr_cid;
>   
> +	/*
> +	 * After shrinking the number of threads or reducing the number
> +	 * of allowed cpus, reduce the value of max_nr_cid so expansion
> +	 * of cid allocation will preserve cache locality if the number
> +	 * of threads or allowed cpus increase again.
> +	 */
> +	max_nr_cid = atomic_read(&mm->max_nr_cid);
> +	while ((allowed_max_nr_cid = min_t(int, READ_ONCE(mm->nr_cpus_allowed),
> +					   atomic_read(&mm->mm_users))),
> +	       max_nr_cid > allowed_max_nr_cid) {
> +		/* atomic_try_cmpxchg loads previous mm->max_nr_cid into max_nr_cid. */
> +		if (atomic_try_cmpxchg(&mm->max_nr_cid, &max_nr_cid, allowed_max_nr_cid)) {
> +			max_nr_cid = allowed_max_nr_cid;
> +			break;
> +		}
> +	}
>   	/* Try to re-use recent cid. This improves cache locality. */
> -	if (!mm_cid_is_unset(cid) && !cpumask_test_and_set_cpu(cid, cidmask))
> +	cid = __this_cpu_read(pcpu_cid->recent_cid);
> +	if (!mm_cid_is_unset(cid) && cid < max_nr_cid &&
> +	    !cpumask_test_and_set_cpu(cid, cidmask))
>   		return cid;
>   	/*
>   	 * Expand cid allocation if the maximum number of concurrency
> @@ -3709,8 +3727,9 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm)
>   	 * and number of threads. Expanding cid allocation as much as
>   	 * possible improves cache locality.
>   	 */
> -	cid = atomic_read(&mm->max_nr_cid);
> +	cid = max_nr_cid;
>   	while (cid < READ_ONCE(mm->nr_cpus_allowed) && cid < atomic_read(&mm->mm_users)) {
> +		/* atomic_try_cmpxchg loads previous mm->max_nr_cid into cid. */
>   		if (!atomic_try_cmpxchg(&mm->max_nr_cid, &cid, cid + 1))
>   			continue;
>   		if (!cpumask_test_and_set_cpu(cid, cidmask))


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Mathieu Desnoyers]

On 2025-02-13 08:25, Gabriele Monaco wrote:
> On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
>> kernel test robot noticed
>> "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
>>
>> [    2.640924][    T0] ------------[ cut here ]------------
>> [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at kernel/workqueue.c:2495
>> __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
>> [    2.642874][    T0] Modules linked in:
>> [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
>> 6.14.0-rc2-00002-g287adf9e9c1f #1
>> [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
>> PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
>> (kernel/workqueue.c:2495 (discriminator 9))
> 
> There seem to be major problems with this configuration, I'm trying to
> understand what's wrong but, for the time being, this patchset is not
> ready for inclusion.

I'm staring at this now, and I'm thinking we could do a simpler change
that would solve your RT issues without having to introduce a dependency
on workqueue.c.

So if the culprit is that task_mm_cid_work() runs for too long on large
many-cpus systems, why not break it up into smaller iterations ?

So rather than iterating on "for_each_possible_cpu", we could simply
break this down into iteration on at most N cpus, so:

tick #1: iteration on CPUs 0 ..   N - 1
tick #2: iteration on CPUs N .. 2*N - 1
...
circling back to 0 when it reaches the number of possible cpus.

This N value could be configurable, e.g. CONFIG_RSEQ_CID_SCAN_BATCH,
with a sane default. An RT system could decide to make that value lower.

Then all we need to do is remember which was that last observed cpu
number in the mm struct, so the next tick picks up from there.

The main downside of this approach compared to scheduling delayed
work in a workqueue is that it depends on having the mm be current when
the scheduler tick happens. But perhaps this is something we could fix
in a different way that does not add a dependency on workqueue. I'm not
sure how though.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH v6 2/3] sched: Move task_mm_cid_work to mm delayed work

[Gabriele Monaco]

On Thu, 2025-02-13 at 12:31 -0500, Mathieu Desnoyers wrote:
> On 2025-02-13 08:25, Gabriele Monaco wrote:
> > On Thu, 2025-02-13 at 14:52 +0800, kernel test robot wrote:
> > > kernel test robot noticed
> > > "WARNING:at_kernel/workqueue.c:#__queue_delayed_work" on:
> > > 
> > > [    2.640924][    T0] ------------[ cut here ]------------
> > > [ 2.641646][ T0] WARNING: CPU: 0 PID: 0 at
> > > kernel/workqueue.c:2495
> > > __queue_delayed_work (kernel/workqueue.c:2495 (discriminator 9))
> > > [    2.642874][    T0] Modules linked in:
> > > [    2.643381][    T0] CPU: 0 UID: 0 PID: 0 Comm: swapper Not
> > > tainted
> > > 6.14.0-rc2-00002-g287adf9e9c1f #1
> > > [    2.644582][    T0] Hardware name: QEMU Standard PC (i440FX +
> > > PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > > [ 2.645943][ T0] RIP: 0010:__queue_delayed_work
> > > (kernel/workqueue.c:2495 (discriminator 9))
> > 
> > There seem to be major problems with this configuration, I'm trying
> > to
> > understand what's wrong but, for the time being, this patchset is
> > not
> > ready for inclusion.
> 
> I'm staring at this now, and I'm thinking we could do a simpler
> change
> that would solve your RT issues without having to introduce a
> dependency
> on workqueue.c.
> 
> So if the culprit is that task_mm_cid_work() runs for too long on
> large
> many-cpus systems, why not break it up into smaller iterations ?
> 
> So rather than iterating on "for_each_possible_cpu", we could simply
> break this down into iteration on at most N cpus, so:
> 
> tick #1: iteration on CPUs 0 ..   N - 1
> tick #2: iteration on CPUs N .. 2*N - 1
> ...
> circling back to 0 when it reaches the number of possible cpus.
> 
> This N value could be configurable, e.g. CONFIG_RSEQ_CID_SCAN_BATCH,
> with a sane default. An RT system could decide to make that value
> lower.
> 
> Then all we need to do is remember which was that last observed cpu
> number in the mm struct, so the next tick picks up from there.
> 
> The main downside of this approach compared to scheduling delayed
> work in a workqueue is that it depends on having the mm be current
> when
> the scheduler tick happens. But perhaps this is something we could
> fix
> in a different way that does not add a dependency on workqueue. I'm
> not
> sure how though.
> 
> Thoughts ?

Mmh, that's indeed neat, what is not so good about this type of task
work is that it's a pure latency, it will happen before scheduling the
task and can't be interrupted.
The only acceptable latency is a bounded one and your idea is going in
that direction.

As you mentioned, this will make the compaction of mm_cid even more
rare and will likely have the test in 3/3 fail even more often, I'm not
sure if this is necessarily a bad thing though, since mm_cid compaction
is mainly aesthetic, so we could just increase the duration of the test
or even add a busy loop inside to make the task more likely to run this
compaction.

I gave a thought about this whole thing, don't take this too seriously,
but what I see essentially flawed in this approach is:
1. task_works are set on tick
2. task_works are run returning to userspace

1. is the issue with frequency and 2. can be mitigated by your idea,
but not essentially solved. What if we (also) did:
1+. set this task_work while switching in
2+. run this task_work while switching out to sleep (i.e. no
preemption)

1+. would make sure all threads have this task_work scheduled at a
certain point (perhaps a bit too much, but we have a periodicity check
in place). 2+. can effectively run the task in a moment when it is not
problematic for the real time response: on a preemptible kernel, as
soon as a task with higher priority is ready to run, it will preempt
the currently running one, the fact current is going to sleep willingly
implies there's no higher priority task ready, so likely no task really
caring about RT response is going to run after.
Not all tasks are ever going to sleep, so we must keep the original
TWA_RESUME in the task_work, especially for those long-running or low-
priority, both unlikely to be RT tasks.

I'm going to try a patch with this CONFIG_RSEQ_CID_SCAN_BATCH and
tuning the test to pass. In the future we can see if those ideas make
sense and perhaps bring them in.

Thanks,
Gabriele

Re: [PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity

[Peter Zijlstra]

On Thu, Feb 13, 2025 at 09:56:17AM -0500, Mathieu Desnoyers wrote:
> On 2025-02-10 10:32, Gabriele Monaco wrote:
> > From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > 
> 
> Peter, Ingo, this patch has been ready for inclusion for a while. The
> rest of this series does not seem to be quite ready yet, but can we at
> least merge this patch into tip ?

Done; stuck it in queue/sched/urgent for the robots. If that passes
I'll push it to tip.