linux-kernel - Re: [mm] f3344adf38: fxmark.hdd_btrfs_DWAL_63

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Date:   Tue, 16 Mar 2021 14:31:53 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Muchun Song <songmuchun@...edance.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Roman Gushchin <guro@...com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com
Subject: Re: [mm] f3344adf38: fxmark.hdd_btrfs_DWAL_63_bufferedio.works/sec
 -52.4% regression

On Mon 15-03-21 13:42:50, Linus Torvalds wrote:
> If somebody can actually figure out what happened there, that would be
> good, but for now it goes into my "archived as a random outlier"
> folder.

This is not something new. We have seen reports like that in the past
already. In many cases there was no apparent reason except for potential
code alignment: e.g. http://lkml.kernel.org/r/20200409135029.GA2072@xsang-OptiPlex-9020
There were some other memcg related which didn't really show any
increase in cache misses or similar. I cannot find those in my notes
right now. There were others like https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/
with analysis http://lkml.kernel.org/r/20201125062445.GA51005@shbuild999.sh.intel.com
which actually pointed to something legit actually.

For this particular report I do not see any real relation to the patch
either.
-- 
Michal Hocko
SUSE Labs