From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Andrew Morton <akpm@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
linux-m68k <linux-m68k@vger.kernel.org>
Subject: Re: BUG: scheduling while atomic: cron/668/0x10c9a0c0
Date: Thu, 2 Jun 2016 13:19:36 +0100 [thread overview]
Message-ID: <20160602121936.GV2527@techsingularity.net> (raw)
In-Reply-To: <0eb1f112-65d4-f2e5-911e-697b21324b9f@suse.cz>
On Thu, Jun 02, 2016 at 02:04:42PM +0200, Vlastimil Babka wrote:
> On 06/02/2016 12:39 PM, Mel Gorman wrote:
> >On Wed, Jun 01, 2016 at 12:01:24PM +0200, Vlastimil Babka wrote:
> >>>Why?
> >>>
> >>>The comment is fine but I do not see why the recalculation would occur.
> >>>
> >>>In the original code, the preferred_zoneref for statistics is calculated
> >>>based on either the supplied nodemask or cpuset_current_mems_allowed during
> >>>the initial attempt. It then relies on the cpuset checks in the slowpath
> >>>to encorce mems_allowed but the preferred zone doesn't change.
> >>>
> >>>With your proposed change, it's possible that the
> >>>preferred_zoneref recalculation points to a zoneref disallowed by
> >>>cpuset_current_mems_sllowed. While it'll be skipped during allocation,
> >>>the statistics will still be against a zone that is potentially outside
> >>>what is allowed.
> >>
> >>Hmm that's true and I was ready to agree. But then I noticed that
> >>gfp_to_alloc_flags() can mask out ALLOC_CPUSET for GFP_ATOMIC. So it's
> >>like a lighter version of the ALLOC_NO_WATERMARKS situation. In that
> >>case it's wrong if we leave ac->preferred_zoneref at a position that has
> >>skipped some zones due to mempolicies?
> >>
> >
> >So both options are wrong then. How about this?
>
> I wonder if the original patch we're fixing was worth all this trouble (and
> more
> for my compaction priority series :), but yeah this should work.
>
I considered that option when the bug report first came in. It was a 2%
hit to the page allocator microbenchmark to revert it. More than I expected
but enough to care. If this causes another problem, I'll revert it as
there will be other options later.
> >---8<---
> >mm, page_alloc: Recalculate the preferred zoneref if the context can ignore memory policies
> >
> >The optimistic fast path may use cpuset_current_mems_allowed instead of
> >of a NULL nodemask supplied by the caller for cpuset allocations. The
> >preferred zone is calculated on this basis for statistic purposes and
> >as a starting point in the zonelist iterator.
> >
> >However, if the context can ignore memory policies due to being atomic or
> >being able to ignore watermarks then the starting point in the zonelist
> >iterator is no longer correct. This patch resets the zonelist iterator in
> >the allocator slowpath if the context can ignore memory policies. This will
> >alter the zone used for statistics but only after it is known that it makes
> >sense for that context. Resetting it before entering the slowpath would
> >potentially allow an ALLOC_CPUSET allocation to be accounted for against
> >the wrong zone. Note that while nodemask is not explicitly set to the
> >original nodemask, it would only have been overwritten if cpuset_enabled()
> >and it was reset before the slowpath was entered.
> >
> >Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
Thanks.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2016-06-02 12:19 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-30 13:13 BUG: scheduling while atomic: cron/668/0x10c9a0c0 (was: Re: mm, page_alloc: avoid looking up the first zone in a zonelist twice) Geert Uytterhoeven
2016-05-30 15:56 ` Mel Gorman
2016-05-30 17:37 ` Geert Uytterhoeven
2016-05-30 18:56 ` Mel Gorman
2016-05-31 9:28 ` Geert Uytterhoeven
2016-05-31 10:13 ` Mel Gorman
2016-05-31 21:44 ` Vlastimil Babka
2016-06-01 9:19 ` Mel Gorman
2016-06-01 10:01 ` BUG: scheduling while atomic: cron/668/0x10c9a0c0 Vlastimil Babka
2016-06-02 10:39 ` Mel Gorman
2016-06-02 12:04 ` Vlastimil Babka
2016-06-02 12:19 ` Mel Gorman [this message]
2016-06-02 18:43 ` Andrew Morton
2016-06-03 3:52 ` Stephen Rothwell
2016-06-03 7:57 ` Geert Uytterhoeven
2016-06-03 8:41 ` Mel Gorman
2016-06-03 9:00 ` Geert Uytterhoeven
2016-06-03 16:35 ` Andrew Morton
2016-06-03 16:46 ` Mel Gorman
2016-06-03 16:49 ` Andrew Morton
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