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

[Breno Leitao <leitao@debian.org>]

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
    
    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>

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);
 		}