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Date:   Wed, 8 May 2019 13:41:52 +0100
From:   Quentin Perret <quentin.perret@arm.com>
To:     Thara Gopinath <thara.gopinath@linaro.org>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Zhang Rui <rui.zhang@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Amit Kachhap <amit.kachhap@gmail.com>,
        viresh kumar <viresh.kumar@linaro.org>,
        Javi Merino <javi.merino@kernel.org>,
        Eduardo Valentin <edubezval@gmail.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Nicolas Dechesne <nicolas.dechesne@linaro.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH V2 1/3] Calculate Thermal Pressure
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Hi Thara,

Sorry for the delayed response.

On Friday 26 Apr 2019 at 10:17:56 (-0400), Thara Gopinath wrote:
> On 04/25/2019 08:45 AM, Vincent Guittot wrote:
> > Do you mean calling a variant of sched_update_thermal_pressure() in
> > update_cpu_capacity() instead of periodic update ?
> > Yes , that should be enough
> 
> Hi,
> 
> I do have some concerns in doing this.
> 1. Updating thermal pressure does involve some calculations for
> accumulating, averaging, decaying etc which in turn could have some
> finite and measurable time spent in the function. I am not sure if this
> delay will be acceptable for all systems during load balancing (I have
> not measured the time involved). We need to decide if this is something
> we can live with.
> 
> 2. More importantly, since update can happen from at least two paths (
> thermal fw and periodic timer in case of this patch series)to ensure
> mutual exclusion,  the update is done under a spin lock. Again calling
> from update_cpu_capacity will involve holding the lock in the load
> balance path which is possible not for the best.
> For me, updating out of load balance minimizes the disruption to
> scheduler on the whole.
> 
> But if there is an over whelming support for updating the statistics
> from the LB , I can move the code.

If I try to clarify my point a little bit, my observation is really that
it's a shame to update the thermal stats often, but to not reflect that
in capacity_of().

So in fact there are two alternatives: 1) do the update only during LB
(which is what I suggested first) to avoid 'useless' work; or 2) reflect
the thermal pressure in the CPU capacity every time the thermal stats
are updated.

And thinking more about it, perhaps 2) is actually a better option? With
this we could try smaller decay periods than the LB interval (which is
most likely useless otherwise) and make sure the capacity considered
during wake-up is up-to-date. This should be a good thing for latency
sensitive tasks I think. (If you consider a task in the Android display
pipeline for example, it needs to run within 16ms or the frame is
missed. So, on wake-up, we'd like to know where the task can run fast
_now_, not according to the capacities the CPUs had 200ms ago or so).

Thoughts ?
Quentin