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Date: Mon, 9 Aug 2021 15:13:49 -0400
From: Nitesh Lal <nilal@...hat.com>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: linux-kernel@...r.kernel.org,
Nicolas Saenz Julienne <nsaenzju@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Christoph Lameter <cl@...ux.com>,
Juri Lelli <juri.lelli@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [patch 4/4] mm: vmstat_refresh: avoid queueing work item if cpu
stats are clean
On Mon, Aug 9, 2021 at 1:34 PM Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Fri, Aug 06, 2021 at 10:47:40PM -0400, Nitesh Lal wrote:
> > Hi Marcelo,
> >
> > On Fri, Jul 30, 2021 at 4:21 PM Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > >
> > > It is not necessary to queue work item to run refresh_vm_stats
> > > on a remote CPU if that CPU has no dirty stats and no per-CPU
> > > allocations for remote nodes.
> >
> > [...]
> >
> > > /*
> > > * The regular update, every sysctl_stat_interval, may come later
> > > @@ -1850,9 +1875,15 @@ int vmstat_refresh(struct ctl_table *tab
> > > * transiently negative values, report an error here if any of
> > > * the stats is negative, so we know to go looking for imbalance.
> > > */
> > > - err = schedule_on_each_cpu(refresh_vm_stats);
> > > - if (err)
> > > - return err;
> > > + get_online_cpus();
> > > + for_each_online_cpu(cpu) {
> > > + if (need_update(cpu) || need_drain_remote_zones(cpu))
> > > + work_on_cpu(cpu, refresh_vm_stats, NULL);
> > > +
> > > + cond_resched();
> > > + }
> > > + put_online_cpus();
> >
> > While testing this patch-set by enabling the debug config options, I
> > ran into the following splat:
> >
> > [ 945.149209] ======================================================
> > [ 945.156111] WARNING: possible circular locking dependency detected
> > [ 945.163013] 5.14.0-rc3+ #2 Tainted: G S
> > [ 945.168754] ------------------------------------------------------
> > [ 945.175655] sysctl/6820 is trying to acquire lock:
> > [ 945.181006] ffff8881679efb00
> > ((work_completion)(&wfc.work)){+.+.}-{0:0}, at:
> > __flush_work+0x5c2/0x8c0
> > [ 945.191332]
> > [ 945.191332] but task is already holding lock:
> > [ 945.197846] ffffffff93b43660 (cpu_hotplug_lock){++++}-{0:0}, at:
> > vmstat_refresh+0x4a/0x450
> > [ 945.207098]
> > [ 945.207098] which lock already depends on the new lock.
> > [ 945.207098]
> > [ 945.216228]
> > [ 945.216228] the existing dependency chain (in reverse order) is:
> > [ 945.224574]
> > [ 945.224574] -> #1 (cpu_hotplug_lock){++++}-{0:0}:
> > [ 945.231488] lock_acquire+0x1d7/0x540
> > [ 945.236157] cpus_read_lock+0x3b/0xc0
> > [ 945.240834] alloc_workqueue+0x884/0xd00
> > [ 945.245800] scsi_host_alloc+0xbdb/0xf60
> > [ 945.250769] megasas_probe_one+0x164/0x800 [megaraid_sas]
> > [ 945.257402] local_pci_probe+0xd8/0x170
> > [ 945.262270] work_for_cpu_fn+0x51/0xa0
> > [ 945.267041] process_one_work+0x960/0x16a0
> > [ 945.272200] worker_thread+0x55e/0xbf0
> > [ 945.276970] kthread+0x371/0x440
> > [ 945.281158] ret_from_fork+0x22/0x30
> > [ 945.285737]
> > [ 945.285737] -> #0 ((work_completion)(&wfc.work)){+.+.}-{0:0}:
> > [ 945.293813] check_prevs_add+0x3fd/0x2470
> > [ 945.298874] __lock_acquire+0x23f7/0x2f80
> > [ 945.303934] lock_acquire+0x1d7/0x540
> > [ 945.308605] __flush_work+0x5e2/0x8c0
> > [ 945.313278] work_on_cpu+0xee/0x140
> > [ 945.317753] vmstat_refresh+0x12f/0x450
> > [ 945.322622] proc_sys_call_handler+0x389/0x500
> > [ 945.328170] new_sync_read+0x3af/0x610
> > [ 945.332942] vfs_read+0x268/0x490
> > [ 945.337225] ksys_read+0xf1/0x1c0
> > [ 945.341511] do_syscall_64+0x42/0x90
> > [ 945.346091] entry_SYSCALL_64_after_hwframe+0x44/0xae
> > [ 945.352318]
> > [ 945.352318] other info that might help us debug this:
> > [ 945.352318]
> > [ 945.361254] Possible unsafe locking scenario:
> > [ 945.361254]
> > [ 945.367863] CPU0 CPU1
> > [ 945.372920] ---- ----
> > [ 945.377976] lock(cpu_hotplug_lock);
> > [ 945.382072]
> > lock((work_completion)(&wfc.work));
> > [ 945.390140] lock(cpu_hotplug_lock);
> > [ 945.397046] lock((work_completion)(&wfc.work));
> > [ 945.402301]
> > [ 945.402301] *** DEADLOCK ***
> > [ 945.402301]
> > [ 945.408909] 1 lock held by sysctl/6820:
> > [ 945.413194] #0: ffffffff93b43660 (cpu_hotplug_lock){++++}-{0:0},
> > at: vmstat_refresh+0x4a/0x450
> > [ 945.422929]
> > [ 945.422929] stack backtrace:
> > [ 945.427793] CPU: 25 PID: 6820 Comm: sysctl Tainted: G S
> > 5.14.0-rc3+ #2
> > [ 945.436540] Hardware name: Dell Inc. PowerEdge R430/0CN7X8, BIOS
> > 2.6.0 10/31/2017
> > [ 945.444886] Call Trace:
> > [ 945.447623] dump_stack_lvl+0x44/0x57
> > [ 945.451720] check_noncircular+0x280/0x320
> > [ 945.456299] ? print_circular_bug.isra.49+0x440/0x440
> > [ 945.461943] ? mark_lock.part.54+0x107/0x1370
> > [ 945.466813] ? mark_lock.part.54+0x107/0x1370
> > [ 945.471683] ? native_sched_clock+0x32/0x130
> > [ 945.476449] ? lock_chain_count+0x20/0x20
> > [ 945.480929] ? sched_clock_cpu+0x151/0x1d0
> > [ 945.485512] check_prevs_add+0x3fd/0x2470
> > [ 945.489999] ? native_sched_clock+0x32/0x130
> > [ 945.494770] ? sched_clock_cpu+0x151/0x1d0
> > [ 945.499347] ? find_held_lock+0x3a/0x1c0
> > [ 945.503731] ? check_irq_usage+0xe10/0xe10
> > [ 945.508306] ? lockdep_lock+0xbf/0x1b0
> > [ 945.512495] ? static_obj+0xc0/0xc0
> > [ 945.516392] ? sched_clock_cpu+0x151/0x1d0
> > [ 945.520975] __lock_acquire+0x23f7/0x2f80
> > [ 945.525461] ? rcu_read_lock_bh_held+0xc0/0xc0
> > [ 945.530430] lock_acquire+0x1d7/0x540
> > [ 945.534523] ? __flush_work+0x5c2/0x8c0
> > [ 945.538811] ? rcu_read_unlock+0x50/0x50
> > [ 945.543196] ? native_sched_clock+0x32/0x130
> > [ 945.547969] ? __queue_work+0x4a3/0xfd0
> > [ 945.552256] ? rcu_read_lock_sched_held+0xaf/0xe0
> > [ 945.557514] __flush_work+0x5e2/0x8c0
> > [ 945.561604] ? __flush_work+0x5c2/0x8c0
> > [ 945.565894] ? queue_delayed_work_on+0x80/0x80
> > [ 945.570853] ? lock_downgrade+0x700/0x700
> > [ 945.575339] ? mark_held_locks+0xb7/0x120
> > [ 945.579829] ? lockdep_hardirqs_on_prepare+0x28f/0x3e0
> > [ 945.585572] ? queue_work_on+0x48/0x80
> > [ 945.589763] ? trace_hardirqs_on+0x32/0x170
> > [ 945.594436] work_on_cpu+0xee/0x140
> > [ 945.598327] ? flush_delayed_work+0xc0/0xc0
> > [ 945.603004] ? work_debug_hint+0x40/0x40
> > [ 945.607388] ? refresh_cpu_vm_stats+0x530/0x530
> > [ 945.612452] ? need_update+0x162/0x210
> > [ 945.616646] vmstat_refresh+0x12f/0x450
> > [ 945.620938] proc_sys_call_handler+0x389/0x500
> > [ 945.625905] ? proc_sys_permission+0x120/0x120
> > [ 945.630872] ? native_sched_clock+0x32/0x130
> > [ 945.635644] new_sync_read+0x3af/0x610
> > [ 945.639838] ? __x64_sys_llseek+0x2e0/0x2e0
> > [ 945.644505] ? native_sched_clock+0x32/0x130
> > [ 945.649290] ? fsnotify+0xf10/0xf10
> > [ 945.653199] vfs_read+0x268/0x490
> > [ 945.656913] ksys_read+0xf1/0x1c0
> > [ 945.660619] ? vfs_write+0x910/0x910
> > [ 945.664615] ? syscall_trace_enter.isra.17+0x18c/0x260
> > [ 945.670369] do_syscall_64+0x42/0x90
> > [ 945.674371] entry_SYSCALL_64_after_hwframe+0x44/0xae
> >
> > Maybe we should replace get/put_online_cpus() with cpu_hotplug/enable_disable()?
>
> schedule_on_each_cpu has a similar pattern and should exhibit
> the same problem.
>
> So perhaps this problem already existed...
> Don't see the warning on a VM.
I triggered this issue on bare-metal, not sure about the VM.
>
> Can you revert the last patch and confirm that it introduces this
> issue?
Yes, on reverting this patch I don't see the splat.
--
Thanks
Nitesh
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