linux-kernel - Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

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Date:   Wed, 26 Jul 2017 23:13:11 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pm@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        smuckle.linux@...il.com, eas-dev@...ts.linaro.org,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [Eas-dev] [PATCH V4 1/3] sched: cpufreq: Allow remote cpufreq callbacks

On Wed, Jul 26, 2017 at 10:50 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 26-07-17, 22:34, Joel Fernandes (Google) wrote:
>> On Wed, Jul 26, 2017 at 2:22 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> > @@ -221,7 +226,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
>> >         sugov_set_iowait_boost(sg_cpu, time, flags);
>> >         sg_cpu->last_update = time;
>> >
>> > -       if (!sugov_should_update_freq(sg_policy, time))
>> > +       if (!sugov_should_update_freq(sg_policy, time, hook->cpu))
>> >                 return;
>>
>> Since with the remote callbacks now possible, isn't it unsafe to
>> modify sg_cpu and sg_policy structures without a lock in
>> sugov_update_single?
>>
>> Unlike sugov_update_shared, we don't acquire any lock in
>> sugov_update_single before updating these structures. Did I miss
>> something?
>
> As Peter already mentioned it earlier, the callbacks are called with
> rq locks held and so sugov_update_single() wouldn't get called in
> parallel for a target CPU.

Ah ok, I have to catch up with that discussion since I missed the
whole thing. Now that you will have me on CC, that shouldn't happen,
thanks and sorry about the noise.

> That's the only race you were worried about ?

Yes. So then in that case, makes sense to move raw_spin_lock in
sugov_update_shared further down? (Just discussing, this point is
independent of your patch), Something like:

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 622eed1b7658..9a6c12fb2c16 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -295,8 +295,6 @@ static void sugov_update_shared(struct
update_util_data *hook, u64 time,

        sugov_get_util(&util, &max);

-       raw_spin_lock(&sg_policy->update_lock);
-
        sg_cpu->util = util;
        sg_cpu->max = max;
        sg_cpu->flags = flags;
@@ -304,6 +302,8 @@ static void sugov_update_shared(struct
update_util_data *hook, u64 time,
        sugov_set_iowait_boost(sg_cpu, time, flags);
        sg_cpu->last_update = time;

+       raw_spin_lock(&sg_policy->update_lock);
+
        if (sugov_should_update_freq(sg_policy, time)) {
                if (flags & SCHED_CPUFREQ_RT_DL)
                        next_f = sg_policy->policy->cpuinfo.max_freq;



thanks,

-Joel