linux-kernel - Re: [PATCH 2/11] cpufreq: governor: Use common mutex for dbs

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Date:	Fri, 5 Feb 2016 04:17:58 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Saravana Kannan <skannan@...eaurora.org>
Subject: Re: [PATCH 2/11] cpufreq: governor: Use common mutex for dbs_data protection

On Fri, Feb 5, 2016 at 4:15 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Fri, Feb 5, 2016 at 4:06 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
>> On Fri, Feb 5, 2016 at 3:59 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>> On 04-02-16, 17:46, Rafael J. Wysocki wrote:
>>>> On Thu, Feb 4, 2016 at 6:09 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>>> > On 04-02-16, 00:16, Rafael J. Wysocki wrote:
>>>> >> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>> >>
>>>> >> Every governor relying on the common code in cpufreq_governor.c
>>>> >> has to provide its own mutex in struct common_dbs_data.  However,
>>>> >> those mutexes are never used at the same time
>>>> >
>>>> > Why do you think so? I thought they can always be used in parallel.
>>>> >
>>>> > Consider 2 or more policies, one can have ondemand as the governor,
>>>> > whereas other one can have conservative.
>>>> >
>>>> > If CPUs go online/offline or if governors are switching in parallel,
>>>> > then cpufreq_governor_dbs() can very much run in parallel for ondemand
>>>> > and conservative.
>>>> >
>>>> > Or am I missing something here ?
>>>>
>>>> Well, so perhaps the changelog is inaccurate.
>>>>
>>>> However, what's wrong with using a single mutex then?
>>>
>>> You are killing the possibility of running the code faster. Consider
>>> this:
>>> - A 16 policy system with N CPUs in every policy (IBM has something
>>>   similar only :) )..
>>> - 4 policies using ondemand, 4 using conservative, 4 using powersave
>>>   and 4 with performance.
>>> - Now if we try to change governors for all of them in parallel, only
>>>   one will be done at a time and others have to wait for this
>>>   BIG-kernel lock.
>>
>> And why is this a big problem, actually?  Why do we want the switching
>> of governors to be that efficient?
>
> In any case, we're talking about having one lock instead of two, mind
> you (out of the tree things don't count), because performance and
> powersave don't even use the code in question.
>
> I'm not really sure how much of a difference that would make on a
> really big system.

And don't we switch governors under policy->rwsem anyway?

Thanks,
Rafael