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

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Message-ID: <CAHk-=wifdJHrfnmwwzPpH-0X6SaZxtdmRWpSNwf8xsXD2iE4dA@mail.gmail.com>
Date:   Mon, 24 Feb 2020 12:09:04 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Feng Tang <feng.tang@...el.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

On Mon, Feb 24, 2020 at 11:24 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I don't know. This does not seem to be a particularly serious load.
> But it does feel like it should be possible to combine the two atomic
> accesses into one, where you don't need to do the refcount thing
> except for the case where sigcount goes from zero to non-zero (and
> back to zero again).

Ok, that looks just as simple as I thought it would be.

TOTALLY UNTESTED patch attached. It may be completely buggy garbage,
but it _looks_ trivial enough. Just make the rule be that "if we have
any user->sigpending cases, we'll get a ref to the user for the first
one, and drop it only when getting rid of the last one".

So it might be worth testing this. But again: I have NOT done so.

There might be some silly reason why this doesn't work because I just
did the tests wrong or missed some case.

Or there might be some subtle reason why it doesn't work because I
didn't think this through properly.

But it _looks_ obvious and simple enough. And it compiles for me. So
maybe it works.

              Linus

View attachment "patch.diff" of type "text/x-patch" (1434 bytes)