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

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Date:   Thu, 18 Jan 2018 10:38:07 +0000
From:   Morten Rasmussen <morten.rasmussen@...s.arm.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Mon, Jan 15, 2018 at 09:26:09AM +0100, Vincent Guittot wrote:
> Le Wednesday 03 Jan 2018 à 10:16:00 (+0100), Vincent Guittot a écrit :
> > Hi Peter,
> > 
> > On 22 December 2017 at 21:42, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Fri, Dec 22, 2017 at 07:56:29PM +0100, Peter Zijlstra wrote:
> > >> Right; but I figured we'd try and do it 'right' and see how horrible it
> > >> is before we try and do funny things.
> > >
> > > So now it should have a 32ms tick for up to .5s when the system goes
> > > completely idle.
> > >
> > > No idea how bad that is..
> > 
> > I have tested your branch but the timer doesn't seem to fire correctly
> > because i can still see blocked load in the use case i have run.
> > I haven't found the reason yet
> 
> Hi Peter,
> 
> With the patch below on top of your branch, the blocked loads are updated and
> decayed regularly. The main differences are:
> - It doesn't use a timer to trig ilb but the tick and when a cpu becomes idle.
>   The main drawback of this solution is that the load is blocked when the
>   system is fully idle with the advantage of not waking up a fully idle
>   system. We have to wait for the next tick or newly idle event for updating
>   blocked load when the system leaves idle stat which can be up to a tick long.
>   If this is too long, we can check for kicking ilb when task wakes up so the
>   blocked load will be updated as soon as the system leaves idle state.
>   The main advantage is that we don't wake up a fully idle system every 32ms to
>   update blocked load that will be not used.
> - I'm working on one more improvement to use nohz_idle_balance in the newly
>   idle case when the system is not overloaded and 
>   (this_rq->avg_idle > sysctl_sched_migration_cost). In this case, we can try to
>   use nohz_idle_balance with NOHZ_STATS_KICK and abort as soon as it exceed
>   this_rq->avg_idle. This will remove some calls to kick_ilb and some wake up
>   of an idle cpus.

This sound like what I meant in my other reply :-)

It seems pointless to have a timer to update PELT if the system is
completely idle, and when it isn't we can piggy back other events to
make the updates happen.