linux-kernel - Re: [PATCH 2/2] mm: compaction: update the cc->nr_migratepages when allocating or freeing the freepages

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Date: Mon, 19 Feb 2024 10:34:26 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
 Zi Yan <ziy@...dia.com>
Cc: mgorman@...hsingularity.net, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: compaction: update the cc->nr_migratepages when
 allocating or freeing the freepages



On 2024/2/12 18:32, Vlastimil Babka wrote:
> On 1/22/24 14:01, Baolin Wang wrote:
>> Currently we will use 'cc->nr_freepages >= cc->nr_migratepages' comparison
>> to ensure that enough freepages are isolated in isolate_freepages(), however
>> it just decreases the cc->nr_freepages without updating cc->nr_migratepages
>> in compaction_alloc(), which will waste more CPU cycles and cause too many
>> freepages to be isolated.
> 
> Hm yeah I guess this can happen with fast_isolate_freepages() if it returns
> with something but not all the freepages that are expected to be needed, and
> then we get to isolate_freepages() again.

Yes.

> 
>> So we should also update the cc->nr_migratepages when allocating or freeing
>> the freepages to avoid isolating excess freepages. And I can see fewer free
>> pages are scanned and isolated when running thpcompact on my Arm64 server:
>>                                         k6.7         k6.7_patched
>> Ops Compaction pages isolated      120692036.00   118160797.00
>> Ops Compaction migrate scanned     131210329.00   154093268.00
>> Ops Compaction free scanned       1090587971.00  1080632536.00
>> Ops Compact scan efficiency               12.03          14.26
>>
>> Moreover, I did not see an obvious latency improvements, this is likely because
>> isolating freepages is not the bottleneck in the thpcompact test case.
>>                                k6.7                  k6.7_patched
>> Amean     fault-both-1      1089.76 (   0.00%)     1080.16 *   0.88%*
>> Amean     fault-both-3      1616.48 (   0.00%)     1636.65 *  -1.25%*
>> Amean     fault-both-5      2266.66 (   0.00%)     2219.20 *   2.09%*
>> Amean     fault-both-7      2909.84 (   0.00%)     2801.90 *   3.71%*
>> Amean     fault-both-12     4861.26 (   0.00%)     4733.25 *   2.63%*
>> Amean     fault-both-18     7351.11 (   0.00%)     6950.51 *   5.45%*
>> Amean     fault-both-24     9059.30 (   0.00%)     9159.99 *  -1.11%*
>> Amean     fault-both-30    10685.68 (   0.00%)    11399.02 *  -6.68%*
>>
>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> ---
>>   mm/compaction.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 066b72b3471a..6c84e3a5b32b 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1779,6 +1779,7 @@ static struct folio *compaction_alloc(struct folio *src, unsigned long data)
>>   	dst = list_entry(cc->freepages.next, struct folio, lru);
>>   	list_del(&dst->lru);
>>   	cc->nr_freepages--;
>> +	cc->nr_migratepages--;
> 
> This is breaking the tracepoint TRACE_EVENT(mm_compaction_migratepages)
> which does
> 
> __entry->nr_failed = cc->nr_migratepages - nr_succeeded;
> 
> and is called after migrate_pages() finishes, so now this will underflow.
> 
> Probably need to get a snapshot of cc->nr_migratepages before calling
> migrate_pages() and then feed that to the tracepoint instead of cc.

Ah, good catch. Will fix in next version. Thanks.