linux-kernel - Re: Is it ok for deferrable timer wakeup the idle cpu?

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Date:	Thu, 23 Jan 2014 13:41:20 +0800
From:	Lei Wen <adrian.wenl@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: Is it ok for deferrable timer wakeup the idle cpu?

On Wed, Jan 22, 2014 at 10:07 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 22 Jan 2014, Lei Wen wrote:
>> Recently I want to do the experiment for cpu isolation over 3.10 kernel.
>> But I find the isolated one is periodically waken up by IPI interrupt.
>>
>> By checking the trace, I find those IPI is generated by add_timer_on,
>> which would calls wake_up_nohz_cpu, and wake up the already idle cpu.
>>
>> With further checking, I find this timer is added by on_demand governor of
>> cpufreq. It would periodically check each cores' state.
>> The problem I see here is cpufreq_governor using INIT_DEFERRABLE_WORK
>> as the tool, while timer is made as deferrable anyway.
>> And what is more that cpufreq checking is very frequent. In my case, the
>> isolated cpu is wakenup by IPI every 5ms.
>>
>> So why kernel need to wake the remote processor when mount the deferrable
>> timer? As per my understanding, we'd better keep cpu as idle when use
>> the deferrable timer.
>
> Indeed, we can avoid the wakeup of the remote cpu when the timer is
> deferrable.

Glad to hear that we could fix this unwanted wakeup.
Do you have related patches already?

>
> Though you really want to figure out why the cpufreq governor is
> arming timers on other cores every 5ms. That smells like an utterly
> stupid approach.

Not sure why cpufreq choose such frequent profiling over each cpu.
As my understanding, since kernel is smp, launching profiler over one cpu
would be enough...

Thanks,
Lei
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