linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Date:	Sun, 3 May 2015 11:24:20 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable &
 enable from context tracking on syscall entry

On Sun, May 3, 2015 at 10:30 AM, Rik van Riel <riel@...hat.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
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