linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Message-ID: <56BBB4B7.2070209@linaro.org>
Date:	Wed, 10 Feb 2016 14:07:51 -0800
From:	Steve Muckle <steve.muckle@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks

On 02/10/2016 01:49 PM, Rafael J. Wysocki wrote:
>>> If done this way, I guess we may pass rq_clock_task(rq) as the time
>>> >> arg to cpufreq_update_util() from there and then the cpu_lock() call
>>> >> I've added to this prototype won't be necessary any more.
>> >
>> > Is it rq_clock_task() or rq_clock()? The former can omit irq time so may
>> > gradually fall behind wall clock time, delaying callbacks in cpufreq.
>
> What matters to us is the difference between the current time and the
> time we previously took a sample and there shouldn't be too much
> difference between the two in that respect.

Sorry, the reference to wall clock time was unnecessary. I just meant it
can lose time, which could cause cpufreq updates to be delayed during
irq heavy periods.

> Both are good enough IMO, but I can update the patch to use rq_clock()
> if that's preferred.

I do believe rq_clock should be used as workloads such as heavy
networking could spend a significant portion of time in interrupts,
skewing rq_clock_task significantly, assuming I understand it correctly.

thanks,
Steve