On 01/14/2015 10:20 PM, Michal Hocko wrote:
On Wed 14-01-15 17:06:59, Vinayak Menon wrote:
[...]
In one such instance, zone_page_state(zone, NR_ISOLATED_FILE)
had returned 14, zone_page_state(zone, NR_INACTIVE_FILE)
returned 92, and GFP_IOFS was set, and this resulted
in too_many_isolated returning true. But one of the CPU's
pageset vm_stat_diff had NR_ISOLATED_FILE as "-14". So the
actual isolated count was zero. As there weren't any more
updates to NR_ISOLATED_FILE and vmstat_update deffered work
had not been scheduled yet, 7 tasks were spinning in the
congestion wait loop for around 4 seconds, in the direct
reclaim path.
Not syncing for such a long time doesn't sound right. I am not familiar
with the vmstat syncing but sysctl_stat_interval is HZ so it should
happen much more often that every 4 seconds.
Though the interval is HZ, since the vmstat_work is declared as a
deferrable work, IIUC the timer trigger can be deferred to the
next non-defferable timer expiry on the CPU which is in idle. This
results in the vmstat syncing on an idle CPU delayed by seconds. May be
in most cases this behavior is fine, except in cases like this. Even in
usual cases were the timer triggers in 1-2 secs, is it fine to let the
tasks in reclaim path wait that long unnecessarily when there isn't any
real congestion?
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