linux-kernel - Re: [PATCH V3 12/13] cpufreq: ondemand: Traverse list of policy_dbs in update_sampling

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Message-ID: <CAJZ5v0iO1yCzW37QG115yPKbooR+1STKQhDxFVYMRS6QXQY8vw@mail.gmail.com>
Date:	Mon, 8 Feb 2016 23:08:48 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Juri Lelli <juri.lelli@....com>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Saravana Kannan <skannan@...eaurora.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Turquette <mturquette@...libre.com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	dietmar.eggemann@....com,
	Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 12/13] cpufreq: ondemand: Traverse list of policy_dbs
 in update_sampling_rate()

On Mon, Feb 8, 2016 at 11:05 PM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Mon, Feb 8, 2016 at 6:20 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> On 08-02-16, 14:32, Rafael J. Wysocki wrote:
>>> The comment still applies.
>>>
>>> Moreover, please extend it to say that this must be called with
>>> dbs_data->mutex held (or it looks racy otherwise).
>>
>> Modified it as:
>>
>> + *
>> + * Simply updating dbs_tuners_int.sampling_rate might not be appropriate here.
>> + * For example, if the original sampling_rate was 1 second and the requested new
>> + * sampling rate is 10 ms because the user needs immediate reaction from
>> + * ondemand governor, otherwise the governor may change the sampling rate too
>> + * late; up to 1 second later.
>
> The "otherwise" doesn't seem to be necessary here.
>
>> + *
>> + * Similar logic applies while increasing the sampling rate. And so we need to
>> + * update it with immediate effect.
>
> Actually, no, it doesn't apply.  If you increase the sampling rate,
> the governor will never be late.  It may be early, but that's fine in
> this case.
>
> It just doesn't hurt to update immediately in this case too.
>
>> + *
>> + * This must be called with dbs_data->mutex held, otherwise traversing
>> + * policy_dbs_list isn't safe.

I really don't know what's wrong with retaining the original paragraph
and adding the above sentence after it.

Thanks,
Rafael