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Message-ID: <5464A84C.1040903@suse.cz>
Date: Thu, 13 Nov 2014 13:47:08 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Joonsoo Kim <iamjoonsoo.kim@....com>
CC: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
Mel Gorman <mgorman@...e.de>,
Michal Nazarewicz <mina86@...a86.com>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Christoph Lameter <cl@...ux.com>,
Rik van Riel <riel@...hat.com>,
David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 5/5] mm, compaction: more focused lru and pcplists draining
On 11/04/2014 01:37 AM, Joonsoo Kim wrote:
> On Mon, Nov 03, 2014 at 09:12:33AM +0100, Vlastimil Babka wrote:
>> On 10/27/2014 08:41 AM, Joonsoo Kim wrote:
>>> On Tue, Oct 07, 2014 at 05:33:39PM +0200, Vlastimil Babka wrote:
>>>
>>> And, I wonder why last_migrated_pfn is set after isolate_migratepages().
>>
>> Not sure I understand your question. With the mistake above, it
>> cannot currently be set at the point isolate_migratepages() is
>> called, so you might question the goto check_drain in the
>> ISOLATE_NONE case, if that's what you are wondering about.
>>
>> When I correct that, it might be set when COMPACT_CLUSTER_MAX pages
>> are isolated and migrated the middle of a pageblock, and then the
>> rest of the pageblock contains no pages that could be isolated, so
>> the last isolate_migratepages() attempt in the pageblock returns
>> with ISOLATE_NONE. Still there were some migrations that produced
>> free pages that should be drained at that point.
>
> To clarify my question, I attach psuedo code that I thought correct.
Sorry for the late reply.
> static int compact_zone()
> {
> unsigned long last_migrated_pfn = 0;
>
> ...
>
> compaction_suitable();
>
> ...
>
> while (compact_finished()) {
> if (!last_migrated_pfn)
> last_migrated_pfn = cc->migrate_pfn - 1;
>
> isolate_migratepages();
> switch case
> migrate_pages();
> ...
>
> check_drain: (at the end of loop)
> do flush and reset last_migrated_pfn if needed
> }
> }
>
> We should record last_migrated_pfn before isolate_migratepages() and
> then compare it with cc->migrate_pfn after isolate_migratepages() to
> know if we moved away from the previous cc->order aligned block.
> Am I missing something?
What about this scenario, with pageblock order:
- record cc->migrate_pfn pointing to pageblock X
- isolate_migratepages() skips the pageblock due to e.g. skip bit, or
the pageblock being a THP already...
- loop to pageblock X+1, last_migrated_pfn is still set to pfn of
pageblock X (more precisely the pfn is (X << pageblock_order) - 1 per
your code, but doesn't matter)
- isolate_migratepages isolates something, but ends up somewhere in the
middle of pageblock due to COMPACT_CLUSTER_MAX
- cc->migrate_pfn points to pageblock X+1 (plus some pages it scanned)
- so it will decide that it has fully migrated pageblock X and it's time
to drain. But the drain is most likely useless - we didn't migrate
anything in pageblock X, we skipped it. And in X+1 we didn't migrate
everything yet, so we should drain only after finishing the other part
of the pageblock.
In short, "last_migrated_pfn" is not "last position of migrate scanner"
but "last block where we *actually* migrated".
I think if you would try to fix the scenario above, you would end up
with something like my patch :)
Vlastimil
> Thanks.
>
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