linux-kernel - Re: [Regerssion] [KSM] KSM CPU overhead in 6.16+ kernel compared to <=6.15 versions ("folio_walk

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Message-ID: <34d27471-80a4-49f8-b6cb-f2e51518d9ea@airmail.cc>
Date: Mon, 13 Oct 2025 17:09:00 +0000
From: craftfever <craftfever@...mail.cc>
To: david@...hat.com
Cc: akpm@...ux-foundation.org, chengming.zhou@...ux.dev,
 craftfever@...ena.io, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 regressions@...ts.linux.dev, xu.xin16@....com.cn
Subject: Re: [Regerssion] [KSM] KSM CPU overhead in 6.16+ kernel compared to
 <=6.15 versions ("folio_walk_start" kernel object overhead)

 > Looking again, no, that's not the case. We do a cond_resched() after 
every page we looked up.
 >
 > Also, b1d3e9bbccb4 was introduced in v6.12 already. Regarding 
folio_walk_start(), also nothing major changed ever since v6.12.
 >
 > Looking at scan_get_next_rmap_item(). I guess we might hold the mmap 
lock for quite a long time (if we're iterating large areas where there 
are no suitable pages mapped -- very large sparse areas).
 >
 > That would explain why we end up calling folio_walk_start() that 
frequently.
 >
 > But nothing really changed in that regard lately in KSM code.
 >
 > What we probably should be doing, is give up the mmap lock after 
scanning a certain size. Or better, switch to per-VMA locks if possible.
 >
 > Also, looking up each address is highly inefficient if we end up having
 > large empty areas. A range-walk function would be much better suited 
for that, so we can just jump over holes completely.
 >
 > But anyhow, nothing seems to have changed ever since 6.15 AFAIKT, so 
I'm not really sure what's going on here. Likely it's unrelated to KSM 
changes.
 >
 > -- Cheers
 >
 > David / dhildenb
 >

I have to make a correction, folio_start_walk is present in "perf top" 
statistics on 6.12-6.15, it just consumes 0.5-1% of kernel time compared 
to 11-14% on 6.16+, where it causes ksmd 100% cpu usage compared <=6.15 
kernels. I understand, that something changed in linked function, that 
affecting KSM behavior. Maybe, you can reproduce it with same settings, 
especially it happens with Chromium apps, there is V8 sandbox with huge 
VM size. Maybe, you could reproduce the problem with the same 
MemoryKSM=yes in user@...rvice, that sets KSM processing for all user 
processes, especially, when Chromium is running. KSM CPU usage really 
differs between 6.12-6.15 and 6.16+. Maybe, it's related to your 
explanation. Thank you.