linux-kernel - Re: [mm, slub] 8ff60eb052: stress-ng.rawpkt.ops_per

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Message-ID: <CAHk-=whJCPchWozS0-4gF8zJ+9SfRv3h7jCSmivr+fs+eJJuZA@mail.gmail.com>
Date:   Wed, 10 Mar 2021 10:17:43 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Lameter <cl@...two.de>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Jann Horn <jannh@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Pekka Enberg <penberg@...nel.org>,
        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, slub] 8ff60eb052: stress-ng.rawpkt.ops_per_sec -47.9% regression

On Tue, Mar 9, 2021 at 10:59 PM Christoph Lameter <cl@...two.de> wrote:
>
> >
> > it really looks like this might well have been very intentional
> > indeed. Or at least very beneficial for _some_ loads.
>
> Yes the thought was that adding an additional page when contention is
> there on the page objects will increase possible concurrency while
> avoiding locks and increase the ability to allocate / free concurrently
> from a multitude of objects.

I wonder if we might have a "try twice before failing" middle ground,
rather than break out on the very first cmpxchg failure (or continue
forever).

Yes, yes, it claims a "Fixes:", but the commit it claims to fix really
does explicitly _mention_ avoiding the loop in the commit message, and
this kernel test robot report very much implies that that original
commit was right, and the "fix" is wrong.

Jann - if you had other loads that showed problems, that would be
worth documenting.

And as mentioned, maybe having a _limited_ retry, rather than a
"continue for as long as there is contention" that clearly regresses
on this (perhaps odd) load?

But for now, I think the thing to do is revert.

         Linus