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

[Dmitry Ilvokhin <d@ilvokhin.com>]

[...]

> 
> 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.

Great find. Thanks for taking the time to track this down.

> 
> 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);
>  		}
>  

Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>