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

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Date:	Fri, 29 Apr 2016 13:50:33 -0700
From:	David Carrillo-Cisneros <davidcc@...gle.com>
To:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.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

That's a possibility, although it will increase the distance between
pmu->add for other perf events and the effective time that CQM
monitoring starts.

On Fri, Apr 29, 2016 at 1:21 PM, Vikas Shivappa
<vikas.shivappa@...ux.intel.com> wrote:
>
>
> On Fri, 29 Apr 2016, David Carrillo-Cisneros wrote:
>
>> (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.
>
>
> CAT did the msr write in switch_to as Peter did not want a new hook to be
> used. Same could be done here.
>
> Thanks,
> Vikas
>
>
>>
>> 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?
>>
>>
>