Re: [PATCH V3 0/3] sched: cpufreq: Allow remote callbacks

From: Rafael J. Wysocki
Date: Thu Jul 13 2017 - 11:17:39 EST

On Thu, Jul 13, 2017 at 8:44 AM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> Hi,
> With Android UI and benchmarks the latency of cpufreq response to
> certain scheduling events can become very critical. Currently, callbacks
> into schedutil are only made from the scheduler if the target CPU of the
> event is the same as the current CPU. This means there are certain
> situations where a target CPU may not run schedutil for some time.
> One testcase to show this behavior is where a task starts running on
> CPU0, then a new task is also spawned on CPU0 by a task on CPU1. If the
> system is configured such that new tasks should receive maximum demand
> initially, this should result in CPU0 increasing frequency immediately.
> Because of the above mentioned limitation though this does not occur.
> This is verified using ftrace with the sample [1] application.
> Maybe the ideal solution is to always allow remote callbacks but that
> has its own challenges:
> o There is no protection required for single CPU per policy case today,
> and adding any kind of locking there, to supply remote callbacks,
> isn't really a good idea.
> o If is local CPU isn't part of the same cpufreq policy as the target
> CPU, then we wouldn't be able to do fast switching at all and have to
> use some kind of bottom half to schedule work on the target CPU to do
> real switching. That may be overkill as well.
> And so this series only allows remote callbacks for target CPUs that
> share the cpufreq policy with the local CPU.
> This series is tested with couple of usecases (Android: hackbench,
> recentfling, galleryfling, vellamo, Ubuntu: hackbench) on ARM hikey
> board (64 bit octa-core, single policy). Only galleryfling showed minor
> improvements, while others didn't had much deviation.
> The reason being that this patchset only targets a corner case, where
> following are required to be true to improve performance and that
> doesn't happen too often with these tests:
> - Task is migrated to another CPU.
> - The task has maximum demand initially, and should take the CPU to
> higher OPPs.
> - And the target CPU doesn't call into schedutil until the next tick.

I don't have any problems with this series at this point, so you can add

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

to the patches.

I can't apply them without ACKs from Peter or Ingo, though.