linux-kernel - Re: [PATCH] cpufreq: conservative: fix requested

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Message-ID: <CADmjqpMG_UVbbmC4Yjveod2q9zHJ6LsxOAYzNZN4NBOr_qhN+Q@mail.gmail.com>
Date:	Fri, 8 Nov 2013 21:29:50 +0200
From:	Stratos Karafotis <skarafotis@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Xiaoguang Chen <chenxg@...vell.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xiaoguang Chen <chenxg.marvell@...il.com>
Subject: Re: [PATCH] cpufreq: conservative: fix requested_freq reduction issue

On Fri, Nov 8, 2013 at 8:16 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 8 November 2013 23:13, Stratos Karafotis <skarafotis@...il.com> wrote:
>> Please let me rephrase my previous post. In some circumstances (depending
>> on freq_step and freq_table values) CPU frequency will never reach to
>> policy->max.
>>
>> For example suppose that (for simplicity values in MHz):
>> policy->max = 1000
>> policy->cur = 800
>> requested_freq = 800
>> freq_target = 300
>>
>> In 'first' iteration, if we return early with this code (because
>> requested_freq will be
>> 1100):
>> if (dbs_info->requested_freq >= policy->max)
>>      return;
>
> That's not correct. At this point requested_freq would have been
> 800 only, and would have increased after this instruction to 1100.
> So, in the first transition we will go to max freq, but not from the
> second.
>
> Though this piece of code is more simplified by the new solution
> I gave.
>

Yes, you are right.

>> CPU freq will never go over 800MHz.
>>
>> I think the current code works correctly.
>> - The requested freq will go to 1100 in first iteration.
>> - __cpufreq_driver_target will change CPU freq to 1000
>> - dbs_cpufreq_notifier will adjust the requested_freq to 1000
>
>> So, I think there is no need for an extra check because of
>> dbs_cpufreq_notifier code.
>
> Now with the new code in place we are correcting requested_freq
> in cs_check_cpu(), then why do we need dbs_cpufreq_notifier()?
>
> What do you think?

I removed the check you proposed in this commit 934dac1ea072 to avoid
the duplicate check in cs_check_cpu and in dbs_cpufreq_notifier.

I agree that we don't need dbs_cpufreq_notifier if we transfer checks in
cs_check_cpu. But I'm not 100% sure if the notifier also covers
other cases and if it can be safely removed.


Stratos Karafotis
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