linux-kernel - Re: [RFC PATCH 1/6] sched/dl: Improve deadline admission control for asymmetric CPU capacities

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Message-ID: <20190507143125.cjfhdxngcugqmko3@queper01-lin>
Date:   Tue, 7 May 2019 15:31:27 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC PATCH 1/6] sched/dl: Improve deadline admission control for
 asymmetric CPU capacities

On Tuesday 07 May 2019 at 16:25:23 (+0200), luca abeni wrote:
> On Tue, 7 May 2019 14:48:52 +0100
> Quentin Perret <quentin.perret@....com> wrote:
> 
> > Hi Luca,
> > 
> > On Monday 06 May 2019 at 06:48:31 (+0200), Luca Abeni wrote:
> > > diff --git a/drivers/base/arch_topology.c
> > > b/drivers/base/arch_topology.c index edfcf8d982e4..646d6d349d53
> > > 100644 --- a/drivers/base/arch_topology.c
> > > +++ b/drivers/base/arch_topology.c
> > > @@ -36,6 +36,7 @@ DEFINE_PER_CPU(unsigned long, cpu_scale) =
> > > SCHED_CAPACITY_SCALE; 
> > >  void topology_set_cpu_scale(unsigned int cpu, unsigned long
> > > capacity) {
> > > +	topology_update_cpu_capacity(cpu, per_cpu(cpu_scale, cpu),
> > > capacity);  
> > 
> > Why is that one needed ? Don't you end up re-building the sched
> > domains after this anyways ?
> 
> If I remember correctly, this function was called at boot time when the
> capacities are assigned to the CPU cores.
> 
> I do not remember if the sched domain was re-built after this call, but
> I admit I do not know this part of the kernel very well...

Right and things moved recently in this area, see bb1fbdd3c3fd
("sched/topology, drivers/base/arch_topology: Rebuild the sched_domain
hierarchy when capacities change")

> This achieved the effect of correctly setting up the "rd_capacity"
> field, but I do not know if there is a better/simpler way to achieve
> the same result :)

OK, that's really an implementation detail, so no need to worry too
much about it at the RFC stage I suppose :-)

Thanks,
Quentin