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

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Date:   Mon, 24 Feb 2020 09:58:09 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Sun, Feb 23, 2020 at 4:33 PM Feng Tang <feng.tang@...el.com> wrote:
>>
>> From the perf c2c data, and the source code checking, the conflicts
>> only happens for root_user.__count, and root_user.sigpending, as
>> all running tasks are accessing this global data for get/put and
>> other operations.
>
> That's odd.
>
> Why? Because those two would be guaranteed to be in the same cacheline
> _after_ you've aligned that user_struct.
>
> So if it were a false sharing issue between those two, it would
> actually get _worse_ with alignment. Those two fields are basically
> next to each other.
>
> But maybe it was straddling a cacheline before, and it caused two
> cache accesses each time?
>
> I find this as confusing as you do.
>
> If it's sigpending vs the __refcount, then we almost always change
> them together. sigpending gets incremented by __sigqueue_alloc() -
> which also does a "get_uid()", and then we decrement it in
> __sigqueue_free() - which also does a "free_uid().
>

One way to verify this is to change the layout of user_struct (or
root_user) to make __count and sigpending fields to be in 2 separate
cache lines explicitly.

Best Regards,
Huang, Ying