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

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Message-ID: <173a99ed-483e-44b4-9784-598464323a39@kernel.org>
Date: Thu, 4 Dec 2025 12:54:56 +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 12/4/25 01:46, Chen Ridong wrote:
> 
> 
> On 2025/12/3 19:33, David Hildenbrand (Red Hat) wrote:
>> 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.
>>
> 
> I prefer tracing nr_to_scan, as it reflects the original target number of pages we intended to scan.

But it's misleading, because we're also tracing "scanned, skipped, 
isolated", and one might wonder how it relates to nr_to_scan?

> Even if nr_to_scan exceeds MAX_LRU_BATCH, we can still deduce that it was effectively capped by
> examining the actual scanned, skipped, or isolated counts. However, if we trace min(nr_to_scan,
> MAX_LRU_BATCH) instead, we would lose visibility into what the original nr_to_scan value was.

Is that really required for the purpose we are tracing here?

-- 
Cheers

David