[PATCH] vmstat: Do not use deferrable delayed work for vmstat_update

[Michal Hocko]

Vinayak Menon has reported that excessive number of tasks was throttled
in the direct reclaim inside too_many_isolated because NR_ISOLATED_FILE
was relatively high compared to NR_INACTIVE_FILE. However it turned out
that the real number of NR_ISOLATED_FILE was 0 and the per-cpu
vm_stat_diff wasn't transfered into the global counter.

vmstat_work which is responsible for the sync is defined as deferrable
delayed work which means that the defined timeout doesn't wake up an
idle CPU. A CPU might stay in an idle state for a long time and general
effort is to keep such a CPU in this state as long as possible which
might lead to all sorts of troubles for vmstat consumers as can be seen
with the excessive direct reclaim throttling.

This patch basically reverts 39bf6270f524 (VM statistics: Make timer
deferrable) but it shouldn't cause any problems for idle CPUs because
only CPUs with an active per-cpu drift are woken up since 7cc36bbddde5
(vmstat: on-demand vmstat workers v8) and CPUs which are idle for a
longer time shouldn't have per-cpu drift.

Fixes: 39bf6270f524 (VM statistics: Make timer deferrable)
Reported-and-debugged-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/vmstat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index c95d6b39ac91..b9b9deec1d54 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1453,7 +1453,7 @@ static void __init start_shepherd_timer(void)
 	int cpu;
 
 	for_each_possible_cpu(cpu)
-		INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
+		INIT_DELAYED_WORK(per_cpu_ptr(&vmstat_work, cpu),
 			vmstat_update);
 
 	if (!alloc_cpumask_var(&cpu_stat_off, GFP_KERNEL))
-- 
2.1.4
-- 
Michal Hocko
SUSE Labs
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