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: <46d26246-5bd5-43f7-b1a4-dc721f717413@airmail.cc>
Date: Mon, 13 Oct 2025 19:54:00 +0000
From: craftfever <craftfever@...mail.cc>
To: david@...hat.com
Cc: 423de7a3-1c62-4e72-8e79-19a6413e420c@...hat.com,
 akpm@...ux-foundation.org, chengming.zhou@...ux.dev, craftfever@...ena.io,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, 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)

Unfortunately, yes I can reproduce it. And I thought that lockups does 
not happen anymore, but I was wrong, I booted today with 6.17.2 updated 
and KSM enabled and whole situation is back. But, it only happens, when 
scanning pages corresponding to a process with huge VM size, like 
Chromium with 1TB of virtual memory. The rest is alright. It's look 
like, that the folio_walk_start called with much higher frequency, than 
in 6.12-6.15 versions. in that version page scanning of huge VM size 
processes is pretty fast and flawless) Right now, when Chromium is 
running, I expecting constant 42% folio_walk_start and 15% 
ksm_scan_thread on 6.17.2 kernel (contrary to 1% folio_walk_start and 
even less ksm_scan_thread in 6.12-6.15). I must admin that whole system 
is not freezing, just Chromium with high CPU usage from ksmd and kernel.

Again, on <=6.15 it's completely oh, and looks like call freq on that 
functions is much more frequent on 6.16+.