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: 7b8da4c ("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>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: leitao

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);
 		}