linux-kernel - Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS

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Date:   Fri, 22 Dec 2017 19:56:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On Fri, Dec 22, 2017 at 03:32:53PM +0100, Vincent Guittot wrote:
> > The only thing I could come up with is running a timer for this :/ That
> > would keep the ILB thing running until all load is decayed (have a patch
> > for that somewhere).
> 
> IMHO running a timer doesn't sound really great

I tend to agree..

> When we have enough activity on the system, the tick and the periodic
> load balance will ensure the update of load of all cpus (including the
> idle cpus) at the load balance period pace.

> But if we don't have enough activity to trig the periodic update
> through ilb or because the system is not overloaded or even almost
> idle, we don't have these periodic update anymore.

> The goal is to do a lazy update of the blocked load to not hurt too
> much power consumption of idle CPUs. When a task wakes up and the
> blocked idle load have not been updated for a while, we trig the
> update of these blocked loads in parallel to the wake up so the data
> will be more accurate for the next events.

> It's already too late for the current wake up but that's not a big
> deal because the wake up path of a light loaded system is mainly
> choosing between previous and current cpu and the load_avg_contrib and
> the utilization will have been updated for next events.

Right; but I figured we'd try and do it 'right' and see how horrible it
is before we try and do funny things.