linux-kernel - Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process

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Message-ID: <CAHk-=wgbR4ocHAOiaj7x+V7dVoYr-mD2N7Y_MRPJ+Q+GohDYeg@mail.gmail.com>
Date:   Mon, 24 Feb 2020 12:47:14 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Feng Tang <feng.tang@...el.com>, Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        kernel test robot <rong.a.chen@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        andi.kleen@...el.com, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
 -5.5% regression

[ Adding a few more people that tend to be involved in signal
handling. Just in case - even if they probably don't care ]

On Mon, Feb 24, 2020 at 12:09 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> TOTALLY UNTESTED patch attached. It may be completely buggy garbage,
> but it _looks_ trivial enough.

I've tested it, and the profiles on the silly microbenchmark look much
nicer. Now it's just the sigpending update shows up, the refcount case
clearly still occasionally happens, but it's now in the noise.

I made slight changes to the __sigqueue_alloc() case to generate
better code: since we now use that atomic_inc_return() anyway, we
might as well then use the value that is returned for the
RLIMIT_SIGPENDING check too, instead of reading it again.

That might avoid another potential cacheline bounce, plus the
generated code just looks better.

Updated (and now slightly tested!) patch attached.

It would be interesting if this is noticeable on your benchmark
numbers. I didn't actually _time_ anything, I just looked at profiles.

But my setup clearly isn't going to see the horrible contention case
anyway, so my timing numbers wouldn't be all that interesting.

             Linus

View attachment "patch.diff" of type "text/x-patch" (1701 bytes)