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

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Message-ID: <YEkVA764JLFuGV9B@kroah.com>
Date:   Wed, 10 Mar 2021 19:50:43 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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

On Wed, Mar 10, 2021 at 10:43:33AM -0800, Linus Torvalds wrote:
> 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.

Thanks for the details, I'll look into reverting it in a future stable
release.

greg k-h