Re: [PATCH v2] mm/vmstat: fix vmstat_shepherd double-scheduling vmstat_update

Michal Hocko
2026-04-10T09:05:21Z

On Thu 09-04-26 05:26:36, Breno Leitao wrote:
> 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
>
> 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().
>
> Fixes: 7b8da4c7f07774 ("vmstat: get rid of the ugly cpu_stat_off variable")
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

I like this much better. Thanks for digging deeper into this.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changes in v2:
> - Instead of changing the timings, do not double-schedule.
> - Link to v1: https://patch.msgid.link/20260401-vmstat-v1-1-b68ce4a35055@debian.org
> ---
> mm/vmstat.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 2370c6fb1fcd6..cc5fdc0d0f298 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -2139,7 +2139,7 @@ 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);
> }
>
>
> ---
> base-commit: cf7c3c02fdd0dfccf4d6611714273dcb538af2cb
> change-id: 20260401-vmstat-048e0feaf344
>
> Best regards,
> --
> Breno Leitao <leitao@debian.org>
>

--
Michal Hocko
SUSE Labs