From: Breno Leitao Date: Thu, 9 Apr 2026 12:26:36 +0000 (-0700) Subject: mm/vmstat: fix vmstat_shepherd double-scheduling vmstat_update X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=2b19bf05719b73f7d04d7d27ec423b459b868852;p=thirdparty%2Fkernel%2Flinux.git 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(). Link: https://lore.kernel.org/20260409-vmstat-v2-1-e9d9a6db08ad@debian.org Fixes: 7b8da4c7f07774 ("vmstat: get rid of the ugly cpu_stat_off variable") Signed-off-by: Breno Leitao Reviewed-by: Vlastimil Babka (SUSE) Acked-by: Michal Hocko Reviewed-by: Dmitry Ilvokhin Cc: Christoph Lameter Cc: David Hildenbrand Cc: Liam Howlett Cc: Lorenzo Stoakes Cc: Mike Rapoport Cc: Shakeel Butt Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- diff --git a/mm/vmstat.c b/mm/vmstat.c index 2370c6fb1fcd..cc5fdc0d0f29 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); }