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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Wed, 16 Mar 2016 09:29:59 +0100
Message-ID: <CAKfTPtA8hJUT4HGXjoyX4=qh9iup3_GUMwuJGgbXtBFGgAPFdQ@mail.gmail.com>
Subject: Re: [PATCH 5/8] sched/cpufreq: pass sched class into cpufreq_update_util
To: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Turquette <mturquette@baylibre.com>,
        "rjw@rjwysocki.net" <rjw@rjwysocki.net>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Steve Muckle <steve.muckle@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Michael Turquette <mturquette+renesas@baylibre.com>
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On 16 March 2016 at 08:41, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Mar 15, 2016 at 03:06:09PM -0700, Michael Turquette wrote:
>> Quoting Peter Zijlstra (2016-03-15 14:25:20)
>> > On Sun, Mar 13, 2016 at 10:22:09PM -0700, Michael Turquette wrote:
>> > > +++ b/include/linux/sched.h
>> > > @@ -2362,15 +2362,25 @@ extern u64 scheduler_tick_max_deferment(void);
>> > >  static inline bool sched_can_stop_tick(void) { return false; }
>> > >  #endif
>> > >
>> > > +enum sched_class_util {
>> > > +     cfs_util,
>> > > +     rt_util,
>> > > +     dl_util,
>> > > +     nr_util_types,
>> > > +};
>> > > +
>> > >  #ifdef CONFIG_CPU_FREQ
>> > >  struct freq_update_hook {
>> > > +     void (*func)(struct freq_update_hook *hook,
>> > > +                  enum sched_class_util sched_class, u64 time,
>> > >                    unsigned long util, unsigned long max);
>> > >  };
>> > >
>> > Have you looked at the asm that generated? At some point you'll start
>> > spilling on the stack and it'll be a god awful mess.
>> >
>>
>> Is your complaint about the enum that I added to the existing function
>> signature, or do you not like the original function signature[0] from
>> Rafael's patch, sans enum?
>
> No, my complaint is more about the call ABI for all our platforms, at
> some point we start passing arguments on the stack instead of through
> registers.
>
> I'm not sure where that starts hurting, but its always a concern when
> adding arguments to functions.

I wonder if it's really worth passing per sched_class request to
sched_util ? sched_util is about selecting a frequency based on the
utilization of the CPU, it only needs a value that reflect the whole
utilization. Can't we sum  (or whatever the formula we want to apply)
utilizations before calling cpufreq_update_util