linux-kernel - Re: [PATCH] cpufreq: schedutil: Update cached "current frequency" when limits change

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Message-ID: <20170714040524.GI352@vireshk-i7>
Date:   Fri, 14 Jul 2017 09:35:24 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: schedutil: Update cached "current frequency"
 when limits change

On 13-07-17, 18:52, Saravana Kannan wrote:
> On 07/11/2017 10:24 PM, Viresh Kumar wrote:
> >On 11-07-17, 19:24, Saravana Kannan wrote:
> >>Currently, the governor calculates the next frequency, set the current CPU
> >>frequency (policy->cur). It also assumes the current CPU frequency doesn't
> >>change if the next frequency isn't calculated again and hence caches the
> >>"current frequency".
> >>
> >>However, this isn't true when CPU min/max frequency limits are changed. So,
> >>there's room for the CPU frequency to get stuck at the wrong level if the
> >>calculated next frequency doesn't change across multiple limits updates.
> >>
> >>Fix this by updating the cached "current frequency" when limits changes the
> >>current CPU frequency.
> >>
> >>Signed-off-by: Saravana Kannan <skannan@...eaurora.org>
> >>---
> >>  kernel/sched/cpufreq_schedutil.c | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >>diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >>index 076a2e3..fe0b2fb 100644
> >>--- a/kernel/sched/cpufreq_schedutil.c
> >>+++ b/kernel/sched/cpufreq_schedutil.c
> >>@@ -226,6 +226,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >>
> >>  	busy = sugov_cpu_is_busy(sg_cpu);
> >>
> >>+	raw_spin_lock(&sg_policy->update_lock);
> >>  	if (flags & SCHED_CPUFREQ_RT_DL) {
> >>  		next_f = policy->cpuinfo.max_freq;
> >>  	} else {
> >>@@ -240,6 +241,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >>  			next_f = sg_policy->next_freq;
> >>  	}
> >>  	sugov_update_commit(sg_policy, time, next_f);
> >>+	raw_spin_unlock(&sg_policy->update_lock);
> >
> >We wouldn't allow locking here until the time we can :)
> >
> 
> Actually, can you clarify why you can't allow locking here?

Because we want this to be quick and there is no need of locking here
as this is getting used only by one CPU. Nothing else :)

-- 
viresh