Re: [PATCH v2] mm: vmscan: fix the page state calculation in too_many_isolated

[Vinayak Menon]

On 01/16/2015 09:19 PM, Michal Hocko wrote:
> On Thu 15-01-15 22:54:20, Vinayak Menon wrote:
> > 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.
>
> I am not sure I understand the above because CPU being idle doesn't
> seem important AFAICS. Anyway I have checked the current code which has
> changed quite recently by 7cc36bbddde5 (vmstat: on-demand vmstat workers
> V8). Let's CC Christoph (the thread starts here:
> http://thread.gmane.org/gmane.linux.kernel.mm/127229).

I will try to explain the exact observations. All the cases which I had 
encountered, had similar symptoms. In one of the cases, it was CPU3 
alone which had not updated the vmstat_diff. This CPU was in idle for 
around 30 secs. When I looked at the tvec base for this CPU, the timer 
associated with vmstat_update had its expiry time less than current 
jiffies. This timer had its deferrable flag set, and was tied to the 
next non-deferrable timer in the list. Since deferrable timers can't 
wake up the CPU, the vmstat sync for this CPU was deferred for a long 
time i.e. till the expiry of next non-deferrable timer. The issue was 
caught because, one of the tasks which was in reclaim path and in the 
congestion_wait loop had an associated watchdog, which resulted in a 
panic after 4secs. So 4 secs is actually the watchdog expiry, and the 
time we can get blocked in the congestion loop can be even more.



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