linux-kernel - Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages

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Date:	Fri, 8 May 2015 16:18:52 -0700
From:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, anton@...bar.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Dan Streetman <ddstreet@...e.org>
Subject: Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc
 reserves if node has no reclaimable pages

On 08.05.2015 [15:47:26 -0700], Andrew Morton wrote:
> On Wed, 06 May 2015 11:28:12 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:
> 
> > On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote:
> > > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote:
> > >>> What I find somewhat worrying though is that we could potentially
> > >>> break the pfmemalloc_watermark_ok() test in situations where
> > >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not
> > >>> a permanently allocated hugepage). In that case, the throttling is
> > >>> supposed to help system recover, and we might be breaking that
> > >>> ability with this patch, no?
> > >>
> > >> Well, if it's transient, we'll skip it this time through, and once there
> > >> are reclaimable pages, we should notice it again.
> > >>
> > >> I'm not familiar enough with this logic, so I'll read through the code
> > >> again soon to see if your concern is valid, as best I can.
> > >
> > > In reviewing the code, I think that transiently unreclaimable zones will
> > > lead to some higher direct reclaim rates and possible contention, but
> > > shouldn't cause any major harm. The likelihood of that situation, as
> > > well, in a non-reserved memory setup like the one I described, seems
> > > exceedingly low.
> > 
> > OK, I guess when a reasonably configured system has nothing to reclaim, 
> > it's already busted and throttling won't change much.
> > 
> > Consider the patch Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> OK, thanks, I'll move this patch into the queue for 4.2-rc1.

Thank you!

> Or is it important enough to merge into 4.1?

I think 4.2 is sufficient, but I wonder now if I should have included a
stable tag? The issue has been around for a while and there's a
relatively easily workaround (use the per-node sysfs files to manually
round-robin around the exhausted node) in older kernels, so I had
decided against it before.

Thanks,
Nish

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