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, 12 Feb 2024 11:32:30 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, 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 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.

> 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.

>  
>  	return dst;
>  }
> @@ -1794,6 +1795,7 @@ static void compaction_free(struct folio *dst, unsigned long data)
>  
>  	list_add(&dst->lru, &cc->freepages);
>  	cc->nr_freepages++;
> +	cc->nr_migratepages++;
>  }
>  
>  /* possible outcome of isolate_migratepages */