Re: [PATCH RFC 2/5] sched/fair: Skip frequency update if CPU about to idle

[Viresh Kumar]

On 28-10-17, 02:59, Joel Fernandes wrote:
> Updating CPU frequency on last dequeue of a CPU is useless. Because the
> utilization since CPU came out of idle can increase till the last dequeue, this
> means we are requesting for a higher frequency before entering idle which is
> not very meaningful or useful. It causes unwanted wakeups of the schedutil
> governor kthread in slow-switch systems resulting in large number of wake ups
> that could have been avoided. In an Android application playing music where the
> music app's thread wakes up and sleeps periodically on an Android device, its
> seen that the frequency increases slightly on the dequeue and is reduced when
> the task wakes up again. This oscillation continues between 300Mhz and 350Mhz,
> and while the task is running, its at 300MHz the whole time. This is pointless.
> Adding to that, these are unnecessary wake ups. Infact most of the time when
> the sugov thread wakes up, all the CPUs are idle - so it can hurt power by
> disturbing the cluster when it is idling.
> 
> This patch prevents a frequency update on the last dequeue. With this the
> number of schedutil governor thread wake ups are reduces more than 2 times
> (1389 -> 527).
> 
> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c  | 25 ++++++++++++++++++++++---
>  kernel/sched/sched.h |  1 +
>  2 files changed, 23 insertions(+), 3 deletions(-)

So you are doing this only for CFS, isn't that required for RT/DL as
well?

Also, this more looks like a policy decision. Will it be better to
put that directly into schedutil? Like this:

        if (cpu_idle())
                "Don't change the freq";

Will something like that work?

-- 
viresh