From: Mel Gorman <mgorman@techsingularity.net>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0 allocations
Date: Fri, 28 Aug 2015 15:12:13 +0100 [thread overview]
Message-ID: <20150828141213.GT12432@techsingularity.net> (raw)
In-Reply-To: <20150828121051.GC5301@dhcp22.suse.cz>
On Fri, Aug 28, 2015 at 02:10:51PM +0200, Michal Hocko wrote:
> On Mon 24-08-15 13:30:15, Mel Gorman wrote:
> > The primary purpose of watermarks is to ensure that reclaim can always
> > make forward progress in PF_MEMALLOC context (kswapd and direct reclaim).
> > These assume that order-0 allocations are all that is necessary for
> > forward progress.
> >
> > High-order watermarks serve a different purpose. Kswapd had no high-order
> > awareness before they were introduced (https://lkml.org/lkml/2004/9/5/9).
>
> lkml.org sucks. Could you plase replace it by something else e.g.
> https://lkml.kernel.org/r/413AA7B2.4000907@yahoo.com.au?
>
Done.
> > This was particularly important when there were high-order atomic requests.
> > The watermarks both gave kswapd awareness and made a reserve for those
> > atomic requests.
> >
> > There are two important side-effects of this. The most important is that
> > a non-atomic high-order request can fail even though free pages are available
> > and the order-0 watermarks are ok. The second is that high-order watermark
> > checks are expensive as the free list counts up to the requested order must
> > be examined.
> >
> > With the introduction of MIGRATE_HIGHATOMIC it is no longer necessary to
> > have high-order watermarks. Kswapd and compaction still need high-order
> > awareness which is handled by checking that at least one suitable high-order
> > page is free.
> >
> > With the patch applied, there was little difference in the allocation
> > failure rates as the atomic reserves are small relative to the number of
> > allocation attempts. The expected impact is that there will never be an
> > allocation failure report that shows suitable pages on the free lists.
> >
> > The one potential side-effect of this is that in a vanilla kernel, the
> > watermark checks may have kept a free page for an atomic allocation. Now,
> > we are 100% relying on the HighAtomic reserves and an early allocation to
> > have allocated them. If the first high-order atomic allocation is after
> > the system is already heavily fragmented then it'll fail.
> >
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
Thanks.
> [...]
> > @@ -2289,7 +2291,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> > {
> > long min = mark;
> > int o;
> > - long free_cma = 0;
> > + const bool atomic = (alloc_flags & ALLOC_HARDER);
>
> I just find the naming a bit confusing. ALLOC_HARDER != __GFP_ATOMIC. RT tasks
> might get access to this reserve as well.
>
I'll just call it alloc_harder then.
> [...]
> > + /* Check at least one high-order page is free */
> > + for (o = order; o < MAX_ORDER; o++) {
> > + struct free_area *area = &z->free_area[o];
> > + int mt;
> > +
> > + if (atomic && area->nr_free)
> > + return true;
>
> Didn't you want
> if (atomic) {
> if (area->nr_free)
> return true;
> continue;
> }
>
That is slightly more efficient so yes, I'll use it. Thanks.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2015-08-28 14:12 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-24 12:09 [PATCH 00/12] Remove zonelist cache and high-order watermark checking v3 Mel Gorman
2015-08-24 12:09 ` [PATCH 01/12] mm, page_alloc: Remove unnecessary parameter from zone_watermark_ok_safe Mel Gorman
2015-08-24 12:09 ` [PATCH 02/12] mm, page_alloc: Remove unnecessary recalculations for dirty zone balancing Mel Gorman
2015-08-24 12:09 ` [PATCH 03/12] mm, page_alloc: Remove unnecessary taking of a seqlock when cpusets are disabled Mel Gorman
2015-08-26 10:25 ` Michal Hocko
2015-08-24 12:09 ` [PATCH 04/12] mm, page_alloc: Only check cpusets when one exists that can be mem-controlled Mel Gorman
2015-08-24 12:37 ` Vlastimil Babka
2015-08-24 13:16 ` Mel Gorman
2015-08-24 20:53 ` Vlastimil Babka
2015-08-25 10:33 ` Mel Gorman
2015-08-25 11:09 ` Vlastimil Babka
2015-08-26 13:41 ` Mel Gorman
2015-08-26 10:46 ` Michal Hocko
2015-08-24 12:09 ` [PATCH 05/12] mm, page_alloc: Remove unecessary recheck of nodemask Mel Gorman
2015-08-25 14:32 ` Vlastimil Babka
2015-08-24 12:09 ` [PATCH 06/12] mm, page_alloc: Use masks and shifts when converting GFP flags to migrate types Mel Gorman
2015-08-25 14:36 ` Vlastimil Babka
2015-08-24 12:09 ` [PATCH 07/12] mm, page_alloc: Distinguish between being unable to sleep, unwilling to sleep and avoiding waking kswapd Mel Gorman
2015-08-24 18:29 ` Mel Gorman
2015-08-25 15:37 ` Vlastimil Babka
2015-08-26 14:45 ` Mel Gorman
2015-08-26 16:24 ` Vlastimil Babka
2015-08-26 18:10 ` Mel Gorman
2015-08-27 9:18 ` Vlastimil Babka
2015-08-25 15:48 ` Vlastimil Babka
2015-08-26 13:05 ` Michal Hocko
2015-09-08 6:49 ` Joonsoo Kim
2015-09-09 12:22 ` Mel Gorman
2015-09-18 6:25 ` Joonsoo Kim
2015-08-24 12:09 ` [PATCH 08/12] mm, page_alloc: Rename __GFP_WAIT to __GFP_RECLAIM Mel Gorman
2015-08-26 12:19 ` Vlastimil Babka
2015-08-24 12:09 ` [PATCH 09/12] mm, page_alloc: Delete the zonelist_cache Mel Gorman
2015-08-24 12:29 ` [PATCH 10/12] mm, page_alloc: Remove MIGRATE_RESERVE Mel Gorman
2015-08-24 12:29 ` [PATCH 11/12] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand Mel Gorman
2015-08-26 12:44 ` Vlastimil Babka
2015-08-26 14:53 ` Michal Hocko
2015-08-26 15:38 ` Mel Gorman
2015-09-08 8:01 ` Joonsoo Kim
2015-09-09 12:32 ` Mel Gorman
2015-09-18 6:38 ` Joonsoo Kim
2015-09-21 10:51 ` Mel Gorman
2015-08-24 12:30 ` [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0 allocations Mel Gorman
2015-08-26 13:42 ` Vlastimil Babka
2015-08-26 14:53 ` Mel Gorman
2015-08-28 12:10 ` Michal Hocko
2015-08-28 14:12 ` Mel Gorman [this message]
2015-09-08 8:26 ` Joonsoo Kim
2015-09-09 12:39 ` Mel Gorman
2015-09-18 6:56 ` Joonsoo Kim
2015-09-21 10:51 ` Mel Gorman
2015-09-30 8:51 ` Vitaly Wool
2015-09-30 13:52 ` Vlastimil Babka
2015-09-30 14:16 ` Vitaly Wool
2015-09-30 14:43 ` Vlastimil Babka
2015-09-30 15:18 ` Mel Gorman
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