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Mon, 06 Jan 2025 05:03:59 -0800 (PST) Received: from localhost ([240f:74:7be:1:de1c:d045:9b:980c]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-72aad81581bsm31380973b3a.2.2025.01.06.05.03.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jan 2025 05:03:59 -0800 (PST) Date: Mon, 6 Jan 2025 22:03:57 +0900 From: Koichiro Den To: Lorenzo Stoakes Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra Subject: Re: [PATCH v2] vmstat: disable vmstat_work on vmstat_cpu_down_prep() Message-ID: <7gd4skn6l4i3liu6cvzmgqlrwxc3rukrnau7lnpyhmfyjuvqwl@gdwgybedp5gs> References: <20241221033321.4154409-1-koichiro.den@canonical.com> <2q7ge6cgzeowqffyn6w6ed4trhaaumv5ubdgud2tsoolen7wpw@4akuomhbacyh> <7ed97096-859e-46d0-8f27-16a2298a8914@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7ed97096-859e-46d0-8f27-16a2298a8914@lucifer.local> X-Stat-Signature: fjuwh8ksfewkf8pzopcknxctzonqgtwc X-Rspamd-Queue-Id: 2F383100051 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1736168648-782655 X-HE-Meta: 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 NQNuWoHX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jan 06, 2025 at 10:52:37AM +0000, Lorenzo Stoakes wrote: > +cc tglx, peterz for insight on CPU hot plug > > On Sat, Jan 04, 2025 at 01:00:17PM +0900, Koichiro Den wrote: > > On Fri, Jan 03, 2025 at 11:33:19PM +0000, Lorenzo Stoakes wrote: > > > On Sat, Dec 21, 2024 at 12:33:20PM +0900, Koichiro Den wrote: > > > > Even after mm/vmstat:online teardown, shepherd may still queue work for > > > > the dying cpu until the cpu is removed from online mask. While it's > > > > quite rare, this means that after unbind_workers() unbinds a per-cpu > > > > kworker, it potentially runs vmstat_update for the dying CPU on an > > > > irrelevant cpu before entering atomic AP states. > > > > When CONFIG_DEBUG_PREEMPT=y, it results in the following error with the > > > > backtrace. > > > > > > > > BUG: using smp_processor_id() in preemptible [00000000] code: \ > > > > kworker/7:3/1702 > > > > caller is refresh_cpu_vm_stats+0x235/0x5f0 > > > > CPU: 0 UID: 0 PID: 1702 Comm: kworker/7:3 Tainted: G > > > > Tainted: [N]=TEST > > > > Workqueue: mm_percpu_wq vmstat_update > > > > Call Trace: > > > > > > > > dump_stack_lvl+0x8d/0xb0 > > > > check_preemption_disabled+0xce/0xe0 > > > > refresh_cpu_vm_stats+0x235/0x5f0 > > > > vmstat_update+0x17/0xa0 > > > > process_one_work+0x869/0x1aa0 > > > > worker_thread+0x5e5/0x1100 > > > > kthread+0x29e/0x380 > > > > ret_from_fork+0x2d/0x70 > > > > ret_from_fork_asm+0x1a/0x30 > > > > > > > > > > > > So, for mm/vmstat:online, disable vmstat_work reliably on teardown and > > > > symmetrically enable it on startup. > > > > > > > > Signed-off-by: Koichiro Den > > > > > > Hi, > > > > > > I observed a warning in my qemu and real hardware, which I bisected to this commit: > > > > > > [ 0.087733] ------------[ cut here ]------------ > > > [ 0.087733] workqueue: work disable count underflowed > > > [ 0.087733] WARNING: CPU: 1 PID: 21 at kernel/workqueue.c:4313 enable_work+0xb5/0xc0 > > > > > > This is: > > > > > > static void work_offqd_enable(struct work_offq_data *offqd) > > > { > > > if (likely(offqd->disable > 0)) > > > offqd->disable--; > > > else > > > WARN_ONCE(true, "workqueue: work disable count underflowed\n"); <-- this line > > > } > > > > > > So (based on this code) presumably an enable is only required if previously > > > disabled, and this code is being called on startup unconditionally without > > > the work having been disabled previously? I'm not hugely familiar with > > > delayed workqueue implementation details. > > > > > > [ 0.087733] Modules linked in: > > > [ 0.087733] CPU: 1 UID: 0 PID: 21 Comm: cpuhp/1 Not tainted 6.13.0-rc4+ #58 > > > [ 0.087733] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014 > > > [ 0.087733] RIP: 0010:enable_work+0xb5/0xc0 > > > [ 0.087733] Code: 6f b8 01 00 74 0f 31 d2 be 01 00 00 00 eb b5 90 0f 0b 90 eb ca c6 05 60 6f b8 01 01 90 48 c7 c7 b0 a9 6e 82 e8 4c a4 fd ff 90 <0f> 0b 90 90 eb d6 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 > > > [ 0.087733] RSP: 0018:ffffc900000cbe30 EFLAGS: 00010092 > > > [ 0.087733] RAX: 0000000000000029 RBX: ffff888263ca9d60 RCX: 0000000000000000 > > > [ 0.087733] RDX: 0000000000000001 RSI: ffffc900000cbce8 RDI: 0000000000000001 > > > [ 0.087733] RBP: ffffc900000cbe30 R08: 00000000ffffdfff R09: ffffffff82b12f08 > > > [ 0.087733] R10: 0000000000000003 R11: 0000000000000002 R12: 00000000000000c4 > > > [ 0.087733] R13: ffffffff81278d90 R14: 0000000000000000 R15: ffff888263c9c648 > > > [ 0.087733] FS: 0000000000000000(0000) GS:ffff888263c80000(0000) knlGS:0000000000000000 > > > [ 0.087733] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > [ 0.087733] CR2: 0000000000000000 CR3: 0000000002a2e000 CR4: 0000000000750ef0 > > > [ 0.087733] PKRU: 55555554 > > > [ 0.087733] Call Trace: > > > [ 0.087733] > > > [ 0.087733] ? enable_work+0xb5/0xc0 > > > [ 0.087733] ? __warn.cold+0x93/0xf2 > > > [ 0.087733] ? enable_work+0xb5/0xc0 > > > [ 0.087733] ? report_bug+0xff/0x140 > > > [ 0.087733] ? handle_bug+0x54/0x90 > > > [ 0.087733] ? exc_invalid_op+0x17/0x70 > > > [ 0.087733] ? asm_exc_invalid_op+0x1a/0x20 > > > [ 0.087733] ? __pfx_vmstat_cpu_online+0x10/0x10 > > > [ 0.087733] ? enable_work+0xb5/0xc0 > > > [ 0.087733] vmstat_cpu_online+0x5c/0x70 > > > [ 0.087733] cpuhp_invoke_callback+0x133/0x440 > > > [ 0.087733] cpuhp_thread_fun+0x95/0x150 > > > [ 0.087733] smpboot_thread_fn+0xd5/0x1d0 > > > [ 0.087734] ? __pfx_smpboot_thread_fn+0x10/0x10 > > > [ 0.087735] kthread+0xc8/0xf0 > > > [ 0.087737] ? __pfx_kthread+0x10/0x10 > > > [ 0.087738] ret_from_fork+0x2c/0x50 > > > [ 0.087739] ? __pfx_kthread+0x10/0x10 > > > [ 0.087740] ret_from_fork_asm+0x1a/0x30 > > > [ 0.087742] > > > [ 0.087742] ---[ end trace 0000000000000000 ]--- > > > > > > > > > > --- > > > > v1: https://lore.kernel.org/all/20241220134234.3809621-1-koichiro.den@canonical.com/ > > > > --- > > > > mm/vmstat.c | 3 ++- > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/mm/vmstat.c b/mm/vmstat.c > > > > index 4d016314a56c..0889b75cef14 100644 > > > > --- a/mm/vmstat.c > > > > +++ b/mm/vmstat.c > > > > @@ -2148,13 +2148,14 @@ static int vmstat_cpu_online(unsigned int cpu) > > > > if (!node_state(cpu_to_node(cpu), N_CPU)) { > > > > node_set_state(cpu_to_node(cpu), N_CPU); > > > > } > > > > + enable_delayed_work(&per_cpu(vmstat_work, cpu)); > > > > > > Probably needs to be 'if disabled' here, as this is invoked on normal > > > startup when the work won't have been disabled? > > > > > > Had a brief look at code and couldn't see how that could be done > > > however... and one would need to be careful about races... Tricky! > > > > > > > > > > > return 0; > > > > } > > > > > > > > static int vmstat_cpu_down_prep(unsigned int cpu) > > > > { > > > > - cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu)); > > > > + disable_delayed_work_sync(&per_cpu(vmstat_work, cpu)); > > > > return 0; > > > > } > > > > > > > > -- > > > > 2.43.0 > > > > > > > > > > > > > > Let me know if you need any more details, .config etc. > > > > > > I noticed this warning on a real box too (in both cases running akpm's > > > mm-unstable branch), FWIW. > > > > Thank you for the report. I was able to reproduce the warning and now > > wonder how I missed it.. My oversight, apologies. > > > > In my current view, the simplest solution would be to make sure a local > > vmstat_work is disabled until vmstat_cpu_online() runs for the cpu, even > > during boot-up. The following patch suppresses the warning: > > > > diff --git a/mm/vmstat.c b/mm/vmstat.c > > index 0889b75cef14..19ceed5d34bf 100644 > > --- a/mm/vmstat.c > > +++ b/mm/vmstat.c > > @@ -2122,10 +2122,14 @@ static void __init start_shepherd_timer(void) > > { > > int cpu; > > > > - for_each_possible_cpu(cpu) > > + for_each_possible_cpu(cpu) { > > INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu), > > vmstat_update); > > > > + /* will be enabled on vmstat_cpu_online */ > > + disable_delayed_work_sync(&per_cpu(vmstat_work, cpu)); > > + } > > + > > schedule_delayed_work(&shepherd, > > round_jiffies_relative(sysctl_stat_interval)); > > } > > > > If you think of a better solution later, please let me know. Otherwise, > > I'll submit a follow-up fix patch with the above diff. > > Thanks, this resolves the problem, but are we sure that _all_ CPUs will > definitely call vmstat_cpu_online()? > > I did a bit of printk output and it seems like this _didn't_ online CPU 0, > presumably the boot CPU which calls this function in the first instance? > > I also see that init_mm_internals() invokes cpuhp_setup_state_nocalls() > explicitly which does _not_ call the callback, though even if it did this > would be too early as it calls start_shepherd_timer() _after_ this anyway. > > So yeah, unless I'm missing something, I think this patch is broken. You're absolutely right, thanks a lot. I also appreciate you testing it, thanks everyone. > > I have added Thomas and Peter to give some insight on the CPU hotplug side. > > It feels like the patch really needs an 'enable if not already enabled' > call in vmstat_cpu_online(). Right. While not fully polished yet, I've tested the following diff: diff --git a/mm/vmstat.c b/mm/vmstat.c index 0889b75cef14..f967aa22392f 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1937,7 +1937,11 @@ static const struct seq_operations vmstat_op = { #endif /* CONFIG_PROC_FS */ #ifdef CONFIG_SMP -static DEFINE_PER_CPU(struct delayed_work, vmstat_work); +struct vmstat_work { + struct delayed_work dwork; + bool enabled; +}; +static DEFINE_PER_CPU(struct vmstat_work, vmstat_work); int sysctl_stat_interval __read_mostly = HZ; static int vmstat_late_init_done; @@ -2015,7 +2019,7 @@ static void vmstat_update(struct work_struct *w) * update worker thread. */ queue_delayed_work_on(smp_processor_id(), mm_percpu_wq, - this_cpu_ptr(&vmstat_work), + &this_cpu_ptr(&vmstat_work)->dwork, round_jiffies_relative(sysctl_stat_interval)); } } @@ -2059,7 +2063,7 @@ void quiet_vmstat(void) if (system_state != SYSTEM_RUNNING) return; - if (!delayed_work_pending(this_cpu_ptr(&vmstat_work))) + if (!delayed_work_pending(&this_cpu_ptr(&vmstat_work)->dwork)) return; if (!need_update(smp_processor_id())) @@ -2091,7 +2095,7 @@ static void vmstat_shepherd(struct work_struct *w) cpus_read_lock(); /* Check processors whose vmstat worker threads have been disabled */ for_each_online_cpu(cpu) { - struct delayed_work *dw = &per_cpu(vmstat_work, cpu); + struct delayed_work *dw = &per_cpu(vmstat_work, cpu).dwork; /* * In kernel users of vmstat counters either require the precise value and @@ -2120,11 +2124,14 @@ static void vmstat_shepherd(struct work_struct *w) static void __init start_shepherd_timer(void) { + struct vmstat_work *vmstat; int cpu; - for_each_possible_cpu(cpu) - INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu), - vmstat_update); + for_each_possible_cpu(cpu) { + vmstat = per_cpu_ptr(&vmstat_work, cpu); + INIT_DEFERRABLE_WORK(&vmstat->dwork, vmstat_update); + vmstat->enabled = true; + } schedule_delayed_work(&shepherd, round_jiffies_relative(sysctl_stat_interval)); @@ -2142,20 +2149,28 @@ static void __init init_cpu_node_state(void) static int vmstat_cpu_online(unsigned int cpu) { + struct vmstat_work *vmstat = per_cpu_ptr(&vmstat_work, cpu); + if (vmstat_late_init_done) refresh_zone_stat_thresholds(); if (!node_state(cpu_to_node(cpu), N_CPU)) { node_set_state(cpu_to_node(cpu), N_CPU); } - enable_delayed_work(&per_cpu(vmstat_work, cpu)); + + if (!vmstat->enabled) { + enable_delayed_work(&vmstat->dwork); + vmstat->enabled = true; + } return 0; } static int vmstat_cpu_down_prep(unsigned int cpu) { - disable_delayed_work_sync(&per_cpu(vmstat_work, cpu)); + struct vmstat_work *vmstat = per_cpu_ptr(&vmstat_work, cpu); + + disable_delayed_work_sync(&vmstat->dwork); return 0; } Lorenzo, and Thomas and Peter if you're available, I'd greatly appreciate any thoughts or feedback on this. Thanks. -Koichiro Den > > > > > Thanks. > > > > -Koichiro