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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Feng Tang <feng.tang@intel.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Jiri Olsa <jolsa@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        kernel test robot <rong.a.chen@intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Jiri Olsa <jolsa@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        "Arnaldo Carvalho de Melo" <acme@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
        Stephane Eranian <eranian@google.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, <lkp@lists.01.org>,
        <andi.kleen@intel.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression
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        <20200205125804.GM14879@hirez.programming.kicks-ass.net>
        <20200221080325.GA67807@shbuild999.sh.intel.com>
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        <CAHk-=wjKFTzfDWjAAabHTZcityeLpHmEQRrKdTuk0f4GWcoohQ@mail.gmail.com>
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Date:   Mon, 24 Feb 2020 09:58:09 +0800
In-Reply-To: <CAHk-=whi87NNOnNXJ6CvyyedmhnS8dZA2YkQQSajvBArH5XOeA@mail.gmail.com>
        (Linus Torvalds's message of "Sun, 23 Feb 2020 17:06:33 -0800")
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Sun, Feb 23, 2020 at 4:33 PM Feng Tang <feng.tang@intel.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