[PATCH] mm, slab: reschedule cache_reap() on the same CPU

[Vlastimil Babka]

cache_reap() is initially scheduled in start_cpu_timer() via
schedule_delayed_work_on(). But then the next iterations are scheduled via
schedule_delayed_work(), i.e. using WORK_CPU_UNBOUND.

Thus since commit ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on
wq_unbound_cpumask CPUs") there is no guarantee the future iterations will run
on the originally intended cpu, although it's still preferred. I was able to
demonstrate this with /sys/module/workqueue/parameters/debug_force_rr_cpu.
IIUC, it may also happen due to migrating timers in nohz context. As a result,
some cpu's would be calling cache_reap() more frequently and others never.

This patch uses schedule_delayed_work_on() with the current cpu when scheduling
the next iteration.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
CC: <stable@xxxxxxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
---
 mm/slab.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/slab.c b/mm/slab.c
index 9095c3945425..a76006aae857 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4074,7 +4074,8 @@ static void cache_reap(struct work_struct *w)
 	next_reap_node();
 out:
 	/* Set up the next iteration */
-	schedule_delayed_work(work, round_jiffies_relative(REAPTIMEOUT_AC));
+	schedule_delayed_work_on(smp_processor_id(), work,
+				round_jiffies_relative(REAPTIMEOUT_AC));
 }
 
 void get_slabinfo(struct kmem_cache *cachep, struct slabinfo *sinfo)
-- 
2.16.3