linux-kernel - Re: [PATCH] mm, vmscan: do not loop on too_many

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Date:   Fri, 21 Jul 2017 16:01:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Mel Gorman <mgorman@...e.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Rik van Riel <riel@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever

On Thu, 20 Jul 2017 08:56:26 +0200 Michal Hocko <mhocko@...nel.org> wrote:
> 
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -1713,9 +1713,15 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> > >  	int file = is_file_lru(lru);
> > >  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> > >  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
> > > +	bool stalled = false;
> > >  
> > >  	while (unlikely(too_many_isolated(pgdat, file, sc))) {
> > > -		congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > +		if (stalled)
> > > +			return 0;
> > > +
> > > +		/* wait a bit for the reclaimer. */
> > > +		schedule_timeout_interruptible(HZ/10);
> > 
> > a) if this task has signal_pending(), this falls straight through
> >    and I suspect the code breaks?
> 
> It will not break. It will return to the allocation path more quickly
> but no over-reclaim will happen and it will/should get throttled there.
> So nothing critical.
> 
> > b) replacing congestion_wait() with schedule_timeout_interruptible()
> >    means this task no longer contributes to load average here and it's
> >    a (slightly) user-visible change.
> 
> you are right. I am not sure it matters but it might be visible.
>  
> > c) msleep_interruptible() is nicer
> > 
> > d) IOW, methinks we should be using msleep() here?
> 
> OK, I do not have objections. Are you going to squash this in or want a
> separate patch explaining all the above?

I'd prefer to have a comment explaining why interruptible sleep is
being used, because that "what if signal_pending()" case is rather a
red flag.

Is it the case that fall-through-if-signal_pending() is the
*preferred* behaviour?  If so, the comment should explain this.  If it
isn't the preferred behaviour then using uninterruptible sleep sounds
better to me, if only because it saves us from having to test a rather
tricky and rare case.