linux-kernel - Re: [x86] d55564cfc2: will-it-scale.per_thread

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Date:   Thu, 7 Jan 2021 19:04:45 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.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: [x86] d55564cfc2: will-it-scale.per_thread_ops -5.8% regression

On Thu, Jan 07, 2021 at 10:47:07AM -0800, Linus Torvalds wrote:

> Now, the "whole entry" is just 8 bytes, so it's possible that it would
> actually be faster to do a copy of the whole thing rather than write
> just the 16 bits. But I got very nervous about it, because I could
> easily see some threaded app actually changing the 'fd' (or the
> 'event' field) in place (ie writing -1 to it as they close and re-use
> it)

BTW, changing 'event' field in place from another thread is going to
be interesting - you have two 16bit values next to each other and
two CPUs modifying those with no exclusion.  Sounds like a recipe
for massive trouble...

Or am I missing something here?