lkml.org 
[lkml]   [2014]   [Nov]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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.
>



\
 
 \ /
  Last update: 2014-11-13 14:21    [W:0.158 / U:0.392 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site