linux-kernel - Re: [PATCH 1/7] sched: Introduce scale-invariant load tracking

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Message-ID: <54353422.3070002@arm.com>
Date:	Wed, 08 Oct 2014 13:54:58 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Yuyang Du <yuyang.du@...el.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>
CC:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"bsegall@...gle.com" <bsegall@...gle.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] sched: Introduce scale-invariant load tracking

Hi Yuyang,

On 08/10/14 01:50, Yuyang Du wrote:
> Hi Morten,
> 
> Sorry for late jumping in.
> 
> The problem seems to be self-evident. But for the implementation to be
> equally attractive it needs to account for every freq change for every task,
> or anything less than that makes it less attractive.
> 
> But this should be very hard. Intel Architecture has limitation to capture all
> the freq changes in software and also the intel_pstate should have no
> notification.

We encountered this missing notification for current frequency with
Intel systems (e.g. i5-3320M) using the intel_pstate driver while
testing this patch-set. The arch_scale_set_curr_freq call in
__cpufreq_notify_transition [[PATCH 2/7] cpufreq: Architecture specific
callback for frequency changes] will not work on such a system.

In our internal testing, we placed arch_scale_set_curr_freq(cpu->cpu,
sample->freq) into intel_pstate_timer_func [intel_pstate.c] to get the
current frequency for a cpu.

The arch_scale_set_max_freq call in cpufreq_set_policy
[drivers/cpufreq/cpufreq.c] still works although the driver exposes the
max turbo pstate and not the max pstate. That's an additional problem
because we don't want to use turbo states for frequency scaling.

> 
> For every task, this makes the updating of the entire queue in load tracking
> more needed, so once again, ping maintainers for the rewrite patches, :)
> 
> Thanks,
> Yuyang
> 

[...]

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