linux-kernel - Re: [PATCH v5] cpufreq: fix governor start/stop race condition

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Message-ID: <CAKohpo=ADC_MGpXFp-QssYS70PHwBBA6qzrWhYGEqZeeaZHLDQ@mail.gmail.com>
Date:	Wed, 19 Jun 2013 08:30:11 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Xiaoguang Chen <chenxg.marvell@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Xiaoguang Chen <chenxg@...vell.com>, cpufreq@...r.kernel.org,
	linux-pm@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	njiang1@...vell.com, zjwu@...vell.com, ylmao@...vell.com
Subject: Re: [PATCH v5] cpufreq: fix governor start/stop race condition

On 19 June 2013 06:50, Xiaoguang Chen <chenxg.marvell@...il.com> wrote:
> 2013/6/19 Rafael J. Wysocki <rjw@...k.pl>:
>> On Thursday, June 13, 2013 05:01:58 PM Xiaoguang Chen wrote:
>>> cpufreq governor stop and start should be kept in sequence.
>>> If not, there will be unexpected behavior, for example:
>>>
>>> we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
>>
>> Please spell cpus as "CPUs".  And please start sequences from capitals.
>
> Ok, thanks for the remind
>
>>
>> [Yes, it *really* is a problem.]

Just wanted to know the reasoning behind it so that I can remind
others about it and then argue :)

>>> the normal sequence is as below:
>>>
>>> 1) Current governor is userspace, one application tries to set
>>> governor to ondemand. it will call __cpufreq_set_policy in which it
>>> will stop userspace governor and then start ondemand governor.
>>
>> Do I think correctly that this is for all CPUs?
>
> From current code design, it is for all CPUs.

Why? This can be for a single cpu (which would eventually force all
others CPUs sharing policy with it).
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