Re: [PATCH] mm/vmstat: spread vmstat_update requeue across the stat interval

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 4/8/26 19:00, Breno Leitao wrote:
> On Wed, Apr 08, 2026 at 08:13:43AM -0700, Breno Leitao wrote:
>> On Wed, Apr 08, 2026 at 12:13:04PM +0200, Vlastimil Babka (SUSE) wrote:
>>
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index 2370c6fb1fcd6..8d53242e7aa66 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -2139,8 +2139,12 @@ static void vmstat_shepherd(struct work_struct *w)
>>                         if (cpu_is_isolated(cpu))
>>                                 continue;
>>
>> -                       if (!delayed_work_pending(dw) && need_update(cpu))
>> +                       if (!delayed_work_pending(dw) && need_update(cpu)) {
>> +                               WARN_ONCE(work_busy(&dw->work) & WORK_BUSY_RUNNING,
>> +                                         "cpu%d: vmstat_update already running, scheduling again\n",
>> +                                         cpu);
>>                                 queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
>> +                       }
>>                 }
>>
>>                 cond_resched();
>>
>> The fix is a one-line change: !delayed_work_pending(dw) → !work_busy(&dw->work)
> 
> In my testing, this race condition occurs more frequently than expected,
> likely due to the timer configurations we've been discussing throughout
> this thread.
> 
> I developed a diagnostic patch to monitor vmstat_update worker scheduling
> frequency, and the results show consistently low values. Avoiding
> rescheduling a worker that is already also reduces the contention on
> stress-ng test case.
> 
> commit d725f0664b70aa5c677215b0fc1abc0117aaf114
> Author: Breno Leitao <leitao@debian.org>
> Date:   Wed Apr 8 09:01:02 2026 -0700
> 
>     mm/vmstat: fix vmstat_shepherd double-scheduling vmstat_update
>     
>     vmstat_shepherd uses delayed_work_pending() to check whether
>     vmstat_update is already scheduled for a given CPU before queuing it.
>     However, delayed_work_pending() only tests WORK_STRUCT_PENDING_BIT,
>     which is cleared the moment a worker thread picks up the work to
>     execute it.
>     
>     This means that while vmstat_update is actively running on a CPU,
>     delayed_work_pending() returns false. If need_update() also returns
>     true at that point (per-cpu counters not yet zeroed mid-flush), the
>     shepherd queues a second invocation with delay=0, causing vmstat_update
>     to run again immediately after finishing.
>     
>     On a 72-CPU system this race is readily observable: before the fix,
>     many CPUs show invocation gaps well below 500 jiffies (the minimum
>     round_jiffies_relative() can produce), with the most extreme cases
>     reaching 0 jiffies—vmstat_update called twice within the same jiffy.
>     
>     Fix this by replacing delayed_work_pending() with work_busy(), which
>     returns non-zero for both WORK_BUSY_PENDING (timer armed or work
>     queued) and WORK_BUSY_RUNNING (work currently executing). The shepherd
>     now correctly skips a CPU in all busy states.
>     
>     After the fix, all sub-jiffy and most sub-100-jiffie gaps disappear.
>     The remaining early invocations have gaps in the 700–999 jiffie range,
>     attributable to round_jiffies_relative() aligning to a nearer
>     jiffie-second boundary rather than to this race.
>     
>     Each spurious vmstat_update invocation has a measurable side effect:
>     refresh_cpu_vm_stats() calls decay_pcp_high() for every zone, which
>     drains idle per-CPU pages back to the buddy allocator via
>     free_pcppages_bulk(), taking the zone spinlock each time. Eliminating
>     the double-scheduling therefore reduces zone lock contention directly.
>     On a 72-CPU stress-ng workload measured with perf lock contention:
>     
>       free_pcppages_bulk contention count:  ~55% reduction
>       free_pcppages_bulk total wait time:   ~57% reduction
>       free_pcppages_bulk max wait time:     ~47% reduction

Great!

>     Note: work_busy() is inherently racy—between the check and the
>     subsequent queue_delayed_work_on() call, vmstat_update can finish
>     execution, leaving the work neither pending nor running. In that
>     narrow window the shepherd can still queue a second invocation.
>     After the fix, this residual race is rare and produces only occasional
>     small gaps, a significant improvement over the systematic
>     double-scheduling seen with delayed_work_pending().
>     
>     Signed-off-by: Breno Leitao <leitao@debian.org>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index d59eff1582547..5489549241b51 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -2156,7 +2156,7 @@ static void vmstat_shepherd(struct work_struct *w)
>  			if (cpu_is_isolated(cpu))
>  				continue;
>  
> -			if (!delayed_work_pending(dw) && need_update(cpu))
> +			if (!work_busy(&dw->work) && need_update(cpu))
>  				queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
>  		}
>