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From: Andy Lutomirski <luto@amacapital.net>
Date: Sun, 3 May 2015 11:24:20 -0700
Message-ID: <CALCETrWj35bCgit7Z516+Pk+GrM_rpvFQsJ5t2fF=5722HjXiQ@mail.gmail.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable &
 enable from context tracking on syscall entry
To: Rik van Riel <riel@redhat.com>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, williams@redhat.com,
        Andrew Lutomirski <luto@kernel.org>, fweisbec@redhat.com,
        Peter Zijlstra <peterz@infradead.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>
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On Sun, May 3, 2015 at 10:30 AM, Rik van Riel <riel@redhat.com> wrote:
> On 05/03/2015 09:23 AM, Mike Galbraith wrote:
>
>> Below are v4.1-rc1-172-g6c3c1eb3c35e + patches measurements.
>>
>> 100M * stat() on isolated cpu
>>
>> NO_HZ_FULL off        inactive     housekeeper    nohz_full
>> real    0m14.266s     0m14.367s    0m20.427s      0m27.921s
>> user    0m1.756s      0m1.553s     0m1.976s       0m10.447s
>> sys     0m12.508s     0m12.769s    0m18.400s      0m17.464s
>> (real)  1.000         1.007        1.431          1.957
>>                                    1.000          1.000
>>
>>                          real      0m20.423s      0m27.930s  +rik 1,2
>>                          user      0m2.072s       0m10.450s
>>                          sys       0m18.304s      0m17.471s
>>                          vs off    1.431          1.957
>>                          vs prev   1.000          1.000
>>
>>                          real      0m20.256s      0m27.803s  +paolo 1,2 (2 missing prototypes)
>>                          user      0m1.884s       0m10.551s
>>                          sys       0m18.353s      0m17.242s
>>                          vs off    1.419          1.948
>>                          vs prev    .991           .995
>>
>>                          real      0m19.122s      0m26.946s  +rik 3
>>                          user      0m1.896s       0m10.292s
>>                          sys       0m17.198s      0m16.644s
>>                          vs off    1.340          1.888
>>                          vs prev   .944            .969
>
> I'm convinced.
>
> Time to try the remote sampling of CPU use statistics, and
> lighten up the RCU overhead of context tracking.
>

I don't understand the remote sampling proposal.  Isn't the whole
point of full nohz to avoid periodically interrupting a busy CPU?  If
what you have in mind is sending IPIs, then that's just as bad, right?

If, on the other hand, you're just going to remotely sample the
in-memory context, that sounds good.

--Andy