linux-kernel - Re: [PATCH 25/32] sched: introduce the finish_arch_pre_lock

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Date:	Fri, 29 Apr 2016 11:40:04 -0700
From:	David Carrillo-Cisneros <davidcc@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
	Tony Luck <tony.luck@...el.com>,
	Stephane Eranian <eranian@...gle.com>,
	Paul Turner <pjt@...gle.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/32] sched: introduce the finish_arch_pre_lock_switch()
 scheduler hook

(Re-sending in plain text)

This hook is used in the following patch in the series to write to
PQR_ASSOC_MSR, a msr that is utilized both by CQM/CMT and by CAT.
Since CAT is not dependent on perf, I created this hook to start CQM
monitoring right after other events start while keeping it independent
of perf. The idea is to have future versions of CAT to also rely on
this hook.

On Fri, Apr 29, 2016 at 11:05 AM, David Carrillo-Cisneros
<davidcc@...gle.com> wrote:
> This hook is used in the following patch in the series to write to
> PQR_ASSOC_MSR, a msr that is utilized both by CQM/CMT and by CAT. Since CAT
> is not dependent on perf, I created this hook to start CQM monitoring right
> after other events start while keeping it independent of perf. The idea is
> to have future versions of CAT to also rely on this hook.
>
> On Fri, Apr 29, 2016 at 1:52 AM Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Thu, Apr 28, 2016 at 09:43:31PM -0700, David Carrillo-Cisneros wrote:
>> > This hook allows architecture specific code to be called at the end of
>> > the task switch and after perf_events' context switch but before the
>> > scheduler lock is released.
>> >
>> > The specific use case in this series is to avoid multiple writes to a
>> > slow
>> > MSR until all functions which modify such register in task switch have
>> > finished.
>>
>> Yeah, no. This really need way more justification. Why can't you use the
>> regular perf sched-in stuff for CQM?