linux-kernel - Re: [PATCH -next] mm: vmscan: correct nr_requested tracing in scan

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Message-ID: <98cbf348-ff21-4c90-af32-b8009c34e5fd@kernel.org>
Date: Wed, 3 Dec 2025 12:33:07 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Chen Ridong <chenridong@...weicloud.com>, akpm@...ux-foundation.org,
 axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 hannes@...xchg.org, mhocko@...nel.org, zhengqi.arch@...edance.com,
 shakeel.butt@...ux.dev, lorenzo.stoakes@...cle.com, yuzhao@...gle.com,
 jaewon31.kim@...sung.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, lujialin4@...wei.com,
 chenridong@...wei.com
Subject: Re: [PATCH -next] mm: vmscan: correct nr_requested tracing in
 scan_folios

On 12/3/25 10:40, Chen Ridong wrote:
> From: Chen Ridong <chenridong@...wei.com>
> 
> When enabling vmscan tracing, it is observed that nr_requested is always
> 4096, which is confusing.
> 
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
>          mm_vmscan_lru_isolate: classzone=3 order=0 nr_requested=4096 ...
> 
> This is because it prints MAX_LRU_BATCH, which is meaningless as it's a
> constant. To fix this, modify it to print nr_to_scan as isolate_lru_folios
> does.
> 
> Fixes: 8c2214fc9a47 ("mm: multi-gen LRU: reuse some legacy trace events")
> Signed-off-by: Chen Ridong <chenridong@...wei.com>
> ---
>   mm/vmscan.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fddd168a9737..8cfafd50a7a8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4601,7 +4601,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>   	count_memcg_events(memcg, item, isolated);
>   	count_memcg_events(memcg, PGREFILL, sorted);
>   	__count_vm_events(PGSCAN_ANON + type, isolated);
> -	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, MAX_LRU_BATCH,
> +	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
>   				scanned, skipped, isolated,

We do that in isolate_lru_folios().

Given that we do

	int remaining = min(nr_to_scan, MAX_LRU_BATCH);

and effectively cap it, I wonder if we would want to trace that capped 
valued instead of MAX_LRU_BATCH.

-- 
Cheers

David