linux-kernel - Re: [PATCH 4.14 27/50] mm, slub: consider rest of partial list if acquire

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Date:   Wed, 10 Mar 2021 10:43:33 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>, Jann Horn <jannh@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4.14 27/50] mm, slub: consider rest of partial list if
 acquire_slab() fails

Just a note to the stable tree: this commit has been reverted
upstream, because it causes a huge performance drop (admittedly on a
load and setup that may not be all that relevant to most people).

It was applied to 4.4, 4.9 and 4.12, because the commit it was marked
as "fixing" is from 2012, but it turns out that the early exit from
the loop in that commit was very much intentional, and very much shows
up on scalability benchmarks.

I don't think this is likely to be a big deal for the stable kernels -
we're basically talking tuning for special cases, and while it is
reverted in my tree now, the "correct" thing to do is likely to be a
bit more flexible than either "exit loop immediately" or "loop for as
long as we have contention".

In practice, most machines probably won't see either case - or it will
at least be rare enough that you can't tell.

The machine that reports a huge performance drop was a multi-socket
machine under fairly extreme conditions, and these contention issues
are often close to exponential - a smaller machine (or a slighly less
extreme load) would never see the issue at all either way.

See

    https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/

for details if you care. I don't think this has to necessarily be
undone in the stable trees, this email is more of an incidental note
just as a heads-up.

                Linus

On Fri, Jan 22, 2021 at 6:14 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> From: Jann Horn <jannh@...gle.com>
>
> commit 8ff60eb052eeba95cfb3efe16b08c9199f8121cf upstream.
>
> acquire_slab() fails if there is contention on the freelist of the page [..]