Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty and writeback pages

From: Michal Hocko <mhocko@kernel.org>
To: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Mel Gorman <mgorman@suse.de>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Rik van Riel <riel@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty and writeback pages
Date: Fri, 30 Oct 2015 09:38:05 +0100	[thread overview]
Message-ID: <20151030083805.GE18429@dhcp22.suse.cz> (raw)
In-Reply-To: <563304B8.6040703@jp.fujitsu.com>

On Fri 30-10-15 14:48:40, KAMEZAWA Hiroyuki wrote:
[...]
> > @@ -3191,8 +3191,23 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> >   		 */
> >   		if (__zone_watermark_ok(zone, order, min_wmark_pages(zone),
> >   				ac->high_zoneidx, alloc_flags, target)) {
> > -			/* Wait for some write requests to complete then retry */
> > -			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/50);
> > +			unsigned long writeback = zone_page_state(zone, NR_WRITEBACK),
> > +				      dirty = zone_page_state(zone, NR_FILE_DIRTY);
> > +
> > +			if (did_some_progress)
> > +				goto retry;
> > +
> > +			/*
> > +			 * If we didn't make any progress and have a lot of
> > +			 * dirty + writeback pages then we should wait for
> > +			 * an IO to complete to slow down the reclaim and
> > +			 * prevent from pre mature OOM
> > +			 */
> > +			if (2*(writeback + dirty) > reclaimable)
> 
> Doesn't this add unnecessary latency if other zones have enough clean memory ?

We know we haven't made any progress the last reclaim round so any zone
with a clean memory is rather unlikely.

-- 
Michal Hocko
SUSE Labs