linux-kernel - Re: [RFC 1/3] mm, oom: refactor oom detection

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Message-ID: <20151030083626.GC18429@dhcp22.suse.cz>
Date:	Fri, 30 Oct 2015 09:36:26 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Hillf Danton <hillf.zj@...baba-inc.com>
Cc:	linux-mm@...ck.org, 'Andrew Morton' <akpm@...ux-foundation.org>,
	'Linus Torvalds' <torvalds@...ux-foundation.org>,
	'Mel Gorman' <mgorman@...e.de>,
	'Johannes Weiner' <hannes@...xchg.org>,
	'Rik van Riel' <riel@...hat.com>,
	'David Rientjes' <rientjes@...gle.com>,
	'Tetsuo Handa' <penguin-kernel@...ove.SAKURA.ne.jp>,
	'LKML' <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/3] mm, oom: refactor oom detection

On Fri 30-10-15 12:10:15, Hillf Danton wrote:
[...]
> > +	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, ac->high_zoneidx, ac->nodemask) {
> > +		unsigned long free = zone_page_state(zone, NR_FREE_PAGES);
> > +		unsigned long reclaimable;
> > +		unsigned long target;
> > +
> > +		reclaimable = zone_reclaimable_pages(zone) +
> > +			      zone_page_state(zone, NR_ISOLATED_FILE) +
> > +			      zone_page_state(zone, NR_ISOLATED_ANON);
> > +		target = reclaimable;
> > +		target -= stall_backoff * (1 + target/MAX_STALL_BACKOFF);
> 
> 		target = reclaimable - stall_backoff * (1 + target/MAX_STALL_BACKOFF);
> 		             = reclaimable - stall_backoff - stall_backoff  * (target/MAX_STALL_BACKOFF);
> 
> then the first stall_backoff looks unreasonable.

First stall_backoff is off by 1 but that shouldn't make any difference.

> I guess you mean
> 		target	= reclaimable - target * (stall_backoff/MAX_STALL_BACKOFF);
> 			= reclaimable - stall_back * (target/MAX_STALL_BACKOFF);

No the reason I used the bias is to converge for MAX_STALL_BACKOFF. If
you have target which is not divisible by MAX_STALL_BACKOFF then the
rounding would get target > 0 and so we wouldn't converge. With the +1
you underflow which is MAX_STALL_BACKOFF in maximum which should be
fixed up by the free memory. Maybe a check for free < MAX_STALL_BACKOFF
would be good but I didn't get that far with this.

[...]

> /*
> > @@ -2734,10 +2730,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> >  		goto retry;
> >  	}
> > 
> > -	/* Any of the zones still reclaimable?  Don't OOM. */
> > -	if (zones_reclaimable)
> > -		return 1;
> > -
> 
> Looks cleanup of zones_reclaimable left.

Removed. Thanks!

-- 
Michal Hocko
SUSE Labs
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