linux-kernel - Re: [PATCH] cpufreq: schedutil: set sg_policy->next

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Message-ID: <20201028082148.zkvcqau4p6xcihoq@vireshk-i7>
Date:   Wed, 28 Oct 2020 13:51:48 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     zhuguangqing83@...il.com
Cc:     rjw@...ysocki.net, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        zhuguangqing <zhuguangqing@...omi.com>
Subject: Re: [PATCH] cpufreq: schedutil: set sg_policy->next_freq to the
 final cpufreq

On 27-10-20, 19:54, zhuguangqing83@...il.com wrote:
> From: zhuguangqing <zhuguangqing@...omi.com>
> 
> In the following code path, next_freq is clamped between policy->min
> and policy->max twice in functions cpufreq_driver_resolve_freq() and
> cpufreq_driver_fast_switch(). For there is no update_lock in the code
> path, policy->min and policy->max may be modified (one or more times),
> so sg_policy->next_freq updated in function sugov_update_next_freq()
> may be not the final cpufreq.

Understood until here, but not sure I followed everything after that.

> Next time when we use
> "if (sg_policy->next_freq == next_freq)" to judge whether to update
> next_freq, we may get a wrong result.
> 
> -> sugov_update_single()
>   -> get_next_freq()
>     -> cpufreq_driver_resolve_freq()
>   -> sugov_fast_switch()
>     -> sugov_update_next_freq()
>     -> cpufreq_driver_fast_switch()
> 
> For example, at first sg_policy->next_freq is 1 GHz, but the final
> cpufreq is 1.2 GHz because policy->min is modified to 1.2 GHz when
> we reached cpufreq_driver_fast_switch(). Then next time, policy->min
> is changed before we reached cpufreq_driver_resolve_freq() and (assume)
> next_freq is 1 GHz, we find "if (sg_policy->next_freq == next_freq)" is
> satisfied so we don't change the cpufreq. Actually we should change
> the cpufreq to 1.0 GHz this time.

FWIW, whenever policy->min/max gets changed, sg_policy->limits_changed
gets set to true by sugov_limits() and the next time schedutil
callback gets called from the scheduler, we will fix the frequency.

And so there shouldn't be any issue here, unless I am missing
something.

-- 
viresh