Re: [Patch] Call cond_resched() at bottom of main look in balance_pgdat()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > =============================================================
> > Subject: [PATCH] Call cond_resched() at bottom of main look in balance_pgdat()
> > From: Larry Woodman <lwoodman@redhat.com>
> > 
> > We are seeing a problem where kswapd gets stuck and hogs the CPU on a
> > small single CPU system when an OOM kill should occur.  When this
> > happens swap space has been exhausted and the pagecache has been shrunk
> > to zero.  Once kswapd gets the CPU it never gives it up because at least
> > one zone is below high.  Adding a single cond_resched() at the end of
> > the main loop in balance_pgdat() fixes the problem by allowing the
> > watchdog and tasks to run and eventually do an OOM kill which frees up
> > the resources.
> > 
> > kosaki note: This seems regression caused by commit bb3ab59683
> > (vmscan: stop kswapd waiting on congestion when the min watermark is
> >  not being met)
> > 
> > Signed-off-by: Larry Woodman <lwoodman@redhat.com>
> > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > ---
> >  mm/vmscan.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9c7e57c..c5c46b7 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2182,6 +2182,7 @@ loop_again:
> >  		 */
> >  		if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
> >  			break;
> > +		cond_resched();
> >  	}
> >  out:
> >  	/*
> > -- 
> > 1.6.5.2
> 
> Kosaki's patch's goal is that kswap doesn't yield cpu if the zone doesn't meet its
> min watermark to avoid failing atomic allocation.
> But this patch could yield kswapd's time slice at any time. 
> Doesn't the patch break your goal in bb3ab59683?

No. it don't break.

Typically, kswapd periodically call shrink_page_list() and it call
cond_resched() even if bb3ab59683 case.
Larry observed very exceptional situation. his system don't have
reclaimable pages at all, then eventually shrink_page_list() was not
called very long time.
His patch only change such very rare situation, I think it's safe.

Thanks.




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