[PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[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 work_struct connected to the
mm_struct rather than in the task context before returning to
userspace.

This work_struct is initialised with the mm and disabled before freeing
it. Its execution is no longer triggered by scheduler ticks: the queuing
of the work happens while returning to userspace in
__rseq_handle_notify_resume, maintaining the checks to avoid running
more frequently than 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 is more predictable with periodic tasks
with short runtime, which may rarely run during a scheduler tick.
Now, the work is always scheduled when the task returns to userspace.

The work is disabled during mmdrop, since the function cannot sleep in
all kernel configurations, we cannot wait for possibly running work
items to terminate. We make sure the mm is valid in case the task is
terminating by reserving it with mmgrab/mmdrop, returning prematurely if
we are really the last user before mmgrab.
This situation is unlikely since we don't schedule the work for exiting
tasks, but we cannot rule it out.

Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 include/linux/mm_types.h |  8 ++++++++
 include/linux/sched.h    |  7 ++++++-
 kernel/rseq.c            |  1 +
 kernel/sched/core.c      | 33 ++++++++++++---------------------
 kernel/sched/sched.h     |  2 --
 5 files changed, 27 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0234f14f2aa6b..e748cf51e0c32 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -889,6 +889,10 @@ struct mm_struct {
 		 * mm nr_cpus_allowed updates.
 		 */
 		raw_spinlock_t cpus_allowed_lock;
+		/*
+		 * @cid_work: Work item to run the mm_cid scan.
+		 */
+		struct work_struct cid_work;
 #endif
 #ifdef CONFIG_MMU
 		atomic_long_t pgtables_bytes;	/* size of all page tables */
@@ -1185,6 +1189,8 @@ enum mm_cid_state {
 	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 +1263,14 @@ 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_WORK(&mm->cid_work, task_mm_cid_work);
 	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_work(&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..2fd65f125153d 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;
@@ -2254,4 +2253,10 @@ static __always_inline void alloc_tag_restore(struct alloc_tag *tag, struct allo
 #define alloc_tag_restore(_tag, _old)		do {} while (0)
 #endif
 
+#ifdef CONFIG_SCHED_MM_CID
+extern void task_queue_mm_cid(struct task_struct *curr);
+#else
+static inline void task_queue_mm_cid(struct task_struct *curr) { }
+#endif
+
 #endif
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 442aba29bc4cf..f8394ebbb6f4d 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 	}
 	if (unlikely(rseq_update_cpu_node_id(t)))
 		goto error;
+	task_queue_mm_cid(t);
 	return;
 
 error:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9aecd914ac691..ee35f9962444b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5663,7 +5663,6 @@ 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);
@@ -10530,22 +10529,16 @@ 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 mm_struct *mm = container_of(work, struct mm_struct, 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)
+	if (!atomic_read(&mm->mm_count))
 		return;
+	mmgrab(mm);
 	old_scan = READ_ONCE(mm->mm_cid_next_scan);
 	next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY);
 	if (!old_scan) {
@@ -10558,9 +10551,9 @@ static void task_mm_cid_work(struct callback_head *work)
 			old_scan = next_scan;
 	}
 	if (time_before(now, old_scan))
-		return;
+		goto out_drop;
 	if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan, next_scan))
-		return;
+		goto out_drop;
 	cidmask = mm_cidmask(mm);
 	/* Clear cids that were not recently used. */
 	for_each_possible_cpu(cpu)
@@ -10572,6 +10565,8 @@ static void task_mm_cid_work(struct callback_head *work)
 	 */
 	for_each_possible_cpu(cpu)
 		sched_mm_cid_remote_clear_weight(mm, cpu, weight);
+out_drop:
+	mmdrop(mm);
 }
 
 void init_sched_mm_cid(struct task_struct *t)
@@ -10584,23 +10579,19 @@ void init_sched_mm_cid(struct task_struct *t)
 		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)
+void task_queue_mm_cid(struct task_struct *curr)
 {
-	struct callback_head *work = &curr->cid_work;
+	struct work_struct *work = &curr->mm->cid_work;
 	unsigned long now = jiffies;
 
-	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) ||
-	    work->next != work)
+	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)))
 		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);
+	schedule_work(work);
 }
 
 void sched_mm_cid_exit_signals(struct task_struct *t)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c8512a9fb0229..37a2e2328283e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3630,7 +3630,6 @@ 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)
@@ -3899,7 +3898,6 @@ 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 */
 
-- 
2.48.1

Re: [PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[Mathieu Desnoyers]

On 2025-02-19 06:31, Gabriele Monaco wrote:
> 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 work_struct connected to the
> mm_struct rather than in the task context before returning to
> userspace.
> 
> This work_struct is initialised with the mm and disabled before freeing
> it. Its execution is no longer triggered by scheduler ticks: the queuing
> of the work happens while returning to userspace in
> __rseq_handle_notify_resume, maintaining the checks to avoid running
> more frequently than 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 is more predictable with periodic tasks
> with short runtime, which may rarely run during a scheduler tick.
> Now, the work is always scheduled when the task returns to userspace.
> 
> The work is disabled during mmdrop, since the function cannot sleep in
> all kernel configurations, we cannot wait for possibly running work
> items to terminate. We make sure the mm is valid in case the task is
> terminating by reserving it with mmgrab/mmdrop, returning prematurely if
> we are really the last user before mmgrab.
> This situation is unlikely since we don't schedule the work for exiting
> tasks, but we cannot rule it out.
> 
> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   include/linux/mm_types.h |  8 ++++++++
>   include/linux/sched.h    |  7 ++++++-
>   kernel/rseq.c            |  1 +
>   kernel/sched/core.c      | 33 ++++++++++++---------------------
>   kernel/sched/sched.h     |  2 --
>   5 files changed, 27 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0234f14f2aa6b..e748cf51e0c32 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -889,6 +889,10 @@ struct mm_struct {
>   		 * mm nr_cpus_allowed updates.
>   		 */
>   		raw_spinlock_t cpus_allowed_lock;
> +		/*
> +		 * @cid_work: Work item to run the mm_cid scan.
> +		 */
> +		struct work_struct cid_work;
>   #endif
>   #ifdef CONFIG_MMU
>   		atomic_long_t pgtables_bytes;	/* size of all page tables */
> @@ -1185,6 +1189,8 @@ enum mm_cid_state {
>   	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 +1263,14 @@ 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_WORK(&mm->cid_work, task_mm_cid_work);
>   	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_work(&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..2fd65f125153d 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;
> @@ -2254,4 +2253,10 @@ static __always_inline void alloc_tag_restore(struct alloc_tag *tag, struct allo
>   #define alloc_tag_restore(_tag, _old)		do {} while (0)
>   #endif
>   
> +#ifdef CONFIG_SCHED_MM_CID
> +extern void task_queue_mm_cid(struct task_struct *curr);
> +#else
> +static inline void task_queue_mm_cid(struct task_struct *curr) { }
> +#endif
> +
>   #endif
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 442aba29bc4cf..f8394ebbb6f4d 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
>   	}
>   	if (unlikely(rseq_update_cpu_node_id(t)))
>   		goto error;
> +	task_queue_mm_cid(t);
>   	return;
>   
>   error:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9aecd914ac691..ee35f9962444b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5663,7 +5663,6 @@ 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);

I agree that this approach is promising, however I am concerned about
the fact that a task running alone on its runqueue (thus without
preemption) for a long time will never recompact mm_cid, and also
will never update its mm_cid field.

So I am tempted to insert this in the sched_tick to cover that scenario:

      rseq_preempt(current);

This would ensure that the task runs __rseq_handle_notify_resume() at
least each tick.

>   	scx_tick(rq);
>   
>   	rq_unlock(rq, &rf);
> @@ -10530,22 +10529,16 @@ 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 mm_struct *mm = container_of(work, struct mm_struct, 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)
> +	if (!atomic_read(&mm->mm_count))
>   		return;
> +	mmgrab(mm);

AFAIU this is racy with respect to re-use of mm struct.

I recommend that you move mmgrab() to task_queue_mm_cid() just before
invoking schedule_work. That way you ensure that the mm count never
reaches 0 while there is work in flight (and therefore guarantee that
the mm is not re-used).

Thanks,

Mathieu

>   	old_scan = READ_ONCE(mm->mm_cid_next_scan);
>   	next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY);
>   	if (!old_scan) {
> @@ -10558,9 +10551,9 @@ static void task_mm_cid_work(struct callback_head *work)
>   			old_scan = next_scan;
>   	}
>   	if (time_before(now, old_scan))
> -		return;
> +		goto out_drop;
>   	if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan, next_scan))
> -		return;
> +		goto out_drop;
>   	cidmask = mm_cidmask(mm);
>   	/* Clear cids that were not recently used. */
>   	for_each_possible_cpu(cpu)
> @@ -10572,6 +10565,8 @@ static void task_mm_cid_work(struct callback_head *work)
>   	 */
>   	for_each_possible_cpu(cpu)
>   		sched_mm_cid_remote_clear_weight(mm, cpu, weight);
> +out_drop:
> +	mmdrop(mm);
>   }
>   
>   void init_sched_mm_cid(struct task_struct *t)
> @@ -10584,23 +10579,19 @@ void init_sched_mm_cid(struct task_struct *t)
>   		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)
> +void task_queue_mm_cid(struct task_struct *curr)
>   {
> -	struct callback_head *work = &curr->cid_work;
> +	struct work_struct *work = &curr->mm->cid_work;
>   	unsigned long now = jiffies;
>   
> -	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)) ||
> -	    work->next != work)
> +	if (!curr->mm || (curr->flags & (PF_EXITING | PF_KTHREAD)))
>   		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);> +	schedule_work(work);
>   }
>   
>   void sched_mm_cid_exit_signals(struct task_struct *t)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c8512a9fb0229..37a2e2328283e 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3630,7 +3630,6 @@ 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)
> @@ -3899,7 +3898,6 @@ 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 */
>   


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

Re: [PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[Gabriele Monaco]

On Wed, 2025-02-19 at 10:13 -0500, Mathieu Desnoyers wrote:
>> On 2025-02-19 06:31, Gabriele Monaco wrote:
>>> > 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 work_struct connected to the
>>> > mm_struct rather than in the task context before returning to
>>> > userspace.
>>> >
>>> > This work_struct is initialised with the mm and disabled before
>>> > freeing
>>> > it. Its execution is no longer triggered by scheduler ticks: the
>>> > queuing
>>> > of the work happens while returning to userspace in
>>> > __rseq_handle_notify_resume, maintaining the checks to avoid
>>> > running
>>> > more frequently than 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 is more predictable with periodic
>>> > tasks
>>> > with short runtime, which may rarely run during a scheduler tick.
>>> > Now, the work is always scheduled when the task returns to
>>> > userspace.
>>> >
>>> > The work is disabled during mmdrop, since the function cannot sleep
>>> > in
>>> > all kernel configurations, we cannot wait for possibly running work
>>> > items to terminate. We make sure the mm is valid in case the task
>>> > is
>>> > terminating by reserving it with mmgrab/mmdrop, returning
>>> > prematurely if
>>> > we are really the last user before mmgrab.
>>> > This situation is unlikely since we don't schedule the work for
>>> > exiting
>>> > tasks, but we cannot rule it out.
>>> >
>>> > Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced
>>> > by mm_cid")
>>> > Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
>>> > ---
>>> > diff --git a/kernel/rseq.c b/kernel/rseq.c
>>> > index 442aba29bc4cf..f8394ebbb6f4d 100644
>>> > --- a/kernel/rseq.c
>>> > +++ b/kernel/rseq.c
>>> > @@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct ksignal
>>> > *ksig, struct pt_regs *regs)
>>> >    }
>>> >    if (unlikely(rseq_update_cpu_node_id(t)))
>>> >    goto error;
>>> > + task_queue_mm_cid(t);
>>> >    return;
>>> >  
>>> >   error:
>>> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> > index 9aecd914ac691..ee35f9962444b 100644
>>> > --- a/kernel/sched/core.c
>>> > +++ b/kernel/sched/core.c
>>> > @@ -5663,7 +5663,6 @@ 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);
>>
>> I agree that this approach is promising, however I am concerned about
>> the fact that a task running alone on its runqueue (thus without
>> preemption) for a long time will never recompact mm_cid, and also
>> will never update its mm_cid field.
>>
>> So I am tempted to insert this in the sched_tick to cover that
>> scenario:
>>
>>       rseq_preempt(current);
>>
>> This would ensure that the task runs __rseq_handle_notify_resume() at
>> least each tick.
>>

Right, I thought about this scenario but forgot to add it in the final patch.
We could have a test doing that: instead of sleeping, the task busy waits.

Does __rseq_handle_notify_resume need to run in this case, besides for the cid compaction, I mean? Otherwise we could again just enqueu
the work from there.

I'll give a shot for both.


>>> >    scx_tick(rq);
>>> >  
>>> >    rq_unlock(rq, &rf);
>>> > @@ -10530,22 +10529,16 @@ 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 mm_struct *mm = container_of(work, struct mm_struct,
>>> > 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)
>>> > + if (!atomic_read(&mm->mm_count))
>>> >    return;
>>> > + mmgrab(mm);
>>
>> AFAIU this is racy with respect to re-use of mm struct.
>>
>> I recommend that you move mmgrab() to task_queue_mm_cid() just before
>> invoking schedule_work. That way you ensure that the mm count never
>> reaches 0 while there is work in flight (and therefore guarantee that
>> the mm is not re-used).
>>


Mmh good point, in that case I think we can still keep on testing the mm_count and return prematurely if it's 1 (we are the only user and the task exited before the work got scheduled).
That would be a safe assumption if we don't get to 0, wouldn't it?

Thanks,
Gabriele

Re: [PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[Mathieu Desnoyers]

On 2025-02-19 11:32, Gabriele Monaco wrote:
> 
> 
> On Wed, 2025-02-19 at 10:13 -0500, Mathieu Desnoyers wrote:
>>> On 2025-02-19 06:31, Gabriele Monaco wrote:
>>>>> 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 work_struct connected to the
>>>>> mm_struct rather than in the task context before returning to
>>>>> userspace.
>>>>>
>>>>> This work_struct is initialised with the mm and disabled before
>>>>> freeing
>>>>> it. Its execution is no longer triggered by scheduler ticks: the
>>>>> queuing
>>>>> of the work happens while returning to userspace in
>>>>> __rseq_handle_notify_resume, maintaining the checks to avoid
>>>>> running
>>>>> more frequently than 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 is more predictable with periodic
>>>>> tasks
>>>>> with short runtime, which may rarely run during a scheduler tick.
>>>>> Now, the work is always scheduled when the task returns to
>>>>> userspace.
>>>>>
>>>>> The work is disabled during mmdrop, since the function cannot sleep
>>>>> in
>>>>> all kernel configurations, we cannot wait for possibly running work
>>>>> items to terminate. We make sure the mm is valid in case the task
>>>>> is
>>>>> terminating by reserving it with mmgrab/mmdrop, returning
>>>>> prematurely if
>>>>> we are really the last user before mmgrab.
>>>>> This situation is unlikely since we don't schedule the work for
>>>>> exiting
>>>>> tasks, but we cannot rule it out.
>>>>>
>>>>> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced
>>>>> by mm_cid")
>>>>> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
>>>>> ---
>>>>> diff --git a/kernel/rseq.c b/kernel/rseq.c
>>>>> index 442aba29bc4cf..f8394ebbb6f4d 100644
>>>>> --- a/kernel/rseq.c
>>>>> +++ b/kernel/rseq.c
>>>>> @@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct ksignal
>>>>> *ksig, struct pt_regs *regs)
>>>>>     }
>>>>>     if (unlikely(rseq_update_cpu_node_id(t)))
>>>>>     goto error;
>>>>> + task_queue_mm_cid(t);
>>>>>     return;
>>>>>   
>>>>>    error:
>>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>>>> index 9aecd914ac691..ee35f9962444b 100644
>>>>> --- a/kernel/sched/core.c
>>>>> +++ b/kernel/sched/core.c
>>>>> @@ -5663,7 +5663,6 @@ 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);
>>>
>>> I agree that this approach is promising, however I am concerned about
>>> the fact that a task running alone on its runqueue (thus without
>>> preemption) for a long time will never recompact mm_cid, and also
>>> will never update its mm_cid field.
>>>
>>> So I am tempted to insert this in the sched_tick to cover that
>>> scenario:
>>>
>>>        rseq_preempt(current);
>>>
>>> This would ensure that the task runs __rseq_handle_notify_resume() at
>>> least each tick.
>>>
> 
> Right, I thought about this scenario but forgot to add it in the final patch.
> We could have a test doing that: instead of sleeping, the task busy waits.
> 
> Does __rseq_handle_notify_resume need to run in this case, besides for the cid compaction, I mean? Otherwise we could again just enqueu
> the work from there.

Yes we need to do both:

- compact cid,
- run __rseq_handle_notify_resume to update the mm_cid.

We we don't care much if compacting the cid is done at some point
and __rseq_handle_notify_resume only updates the mm_cid field on
the following tick.

So enqueuing the work is not sufficient there, I would really
issue rseq_preempt(current) which makes sure a busy thread both
triggers cid compaction *and* gets its mm_cid updated.

> 
> I'll give a shot for both.
> 
> 
>>>>>     scx_tick(rq);
>>>>>   
>>>>>     rq_unlock(rq, &rf);
>>>>> @@ -10530,22 +10529,16 @@ 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 mm_struct *mm = container_of(work, struct mm_struct,
>>>>> 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)
>>>>> + if (!atomic_read(&mm->mm_count))
>>>>>     return;
>>>>> + mmgrab(mm);
>>>
>>> AFAIU this is racy with respect to re-use of mm struct.
>>>
>>> I recommend that you move mmgrab() to task_queue_mm_cid() just before
>>> invoking schedule_work. That way you ensure that the mm count never
>>> reaches 0 while there is work in flight (and therefore guarantee that
>>> the mm is not re-used).
>>>
> 
> 
> Mmh good point, in that case I think we can still keep on testing the mm_count and return prematurely if it's 1 (we are the only user and the task exited before the work got scheduled).
> That would be a safe assumption if we don't get to 0, wouldn't it?

Yes, although don't forget the mmdrop in that case ;)

Thanks,

Mathieu


> 
> Thanks,
> Gabriele
> 


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

Re: [PATCH v7 1/2] sched: Move task_mm_cid_work to mm work_struct

[Gabriele Monaco]

On Wed, 2025-02-19 at 12:08 -0500, Mathieu Desnoyers wrote:
> On 2025-02-19 11:32, Gabriele Monaco wrote:
> > 
> > 
> > On Wed, 2025-02-19 at 10:13 -0500, Mathieu Desnoyers wrote:
> > > > On 2025-02-19 06:31, Gabriele Monaco wrote:
> > > > > > 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 work_struct connected to
> > > > > > the
> > > > > > mm_struct rather than in the task context before returning
> > > > > > to
> > > > > > userspace.
> > > > > > 
> > > > > > This work_struct is initialised with the mm and disabled
> > > > > > before
> > > > > > freeing
> > > > > > it. Its execution is no longer triggered by scheduler
> > > > > > ticks: the
> > > > > > queuing
> > > > > > of the work happens while returning to userspace in
> > > > > > __rseq_handle_notify_resume, maintaining the checks to
> > > > > > avoid
> > > > > > running
> > > > > > more frequently than 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 is more predictable with
> > > > > > periodic
> > > > > > tasks
> > > > > > with short runtime, which may rarely run during a scheduler
> > > > > > tick.
> > > > > > Now, the work is always scheduled when the task returns to
> > > > > > userspace.
> > > > > > 
> > > > > > The work is disabled during mmdrop, since the function
> > > > > > cannot sleep
> > > > > > in
> > > > > > all kernel configurations, we cannot wait for possibly
> > > > > > running work
> > > > > > items to terminate. We make sure the mm is valid in case
> > > > > > the task
> > > > > > is
> > > > > > terminating by reserving it with mmgrab/mmdrop, returning
> > > > > > prematurely if
> > > > > > we are really the last user before mmgrab.
> > > > > > This situation is unlikely since we don't schedule the work
> > > > > > for
> > > > > > exiting
> > > > > > tasks, but we cannot rule it out.
> > > > > > 
> > > > > > Fixes: 223baf9d17f2 ("sched: Fix performance regression
> > > > > > introduced
> > > > > > by mm_cid")
> > > > > > Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> > > > > > ---
> > > > > > diff --git a/kernel/rseq.c b/kernel/rseq.c
> > > > > > index 442aba29bc4cf..f8394ebbb6f4d 100644
> > > > > > --- a/kernel/rseq.c
> > > > > > +++ b/kernel/rseq.c
> > > > > > @@ -419,6 +419,7 @@ void __rseq_handle_notify_resume(struct
> > > > > > ksignal
> > > > > > *ksig, struct pt_regs *regs)
> > > > > >     }
> > > > > >     if (unlikely(rseq_update_cpu_node_id(t)))
> > > > > >     goto error;
> > > > > > + task_queue_mm_cid(t);
> > > > > >     return;
> > > > > >   
> > > > > >    error:
> > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > > > index 9aecd914ac691..ee35f9962444b 100644
> > > > > > --- a/kernel/sched/core.c
> > > > > > +++ b/kernel/sched/core.c
> > > > > > @@ -5663,7 +5663,6 @@ 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);
> > > > 
> > > > I agree that this approach is promising, however I am concerned
> > > > about
> > > > the fact that a task running alone on its runqueue (thus
> > > > without
> > > > preemption) for a long time will never recompact mm_cid, and
> > > > also
> > > > will never update its mm_cid field.
> > > > 
> > > > So I am tempted to insert this in the sched_tick to cover that
> > > > scenario:
> > > > 
> > > >        rseq_preempt(current);
> > > > 
> > > > This would ensure that the task runs
> > > > __rseq_handle_notify_resume() at
> > > > least each tick.
> > > > 
> > 
> > Right, I thought about this scenario but forgot to add it in the
> > final patch.
> > We could have a test doing that: instead of sleeping, the task busy
> > waits.
> > 
> > Does __rseq_handle_notify_resume need to run in this case, besides
> > for the cid compaction, I mean? Otherwise we could again just
> > enqueu
> > the work from there.
> 
> Yes we need to do both:
> 
> - compact cid,
> - run __rseq_handle_notify_resume to update the mm_cid.
> 
> We we don't care much if compacting the cid is done at some point
> and __rseq_handle_notify_resume only updates the mm_cid field on
> the following tick.
> 
> So enqueuing the work is not sufficient there, I would really
> issue rseq_preempt(current) which makes sure a busy thread both
> triggers cid compaction *and* gets its mm_cid updated.
> 

Sure, will do.

I've been trying to test this scenario but it's quite hard to achieve.
I set all threads to FIFO and highest priority, removed all system
calls from the leader thread (even the ones to wait for other threads)
and replaced the usleep with a busy wait, running on a VM so not sure
if interrupts can bother. The test still passes..

Anyway it seems a reasonable situation to happens and I guess it won't
hurt to just run an rseq_preempt in the tick.

Testing and sending V8 without touching the selftest.

Thanks,
Gabriele