linux-kernel - Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities

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Message-ID: <20200408142313.j6yfv6s3i5pzs5wv@e107158-lin.cambridge.arm.com>
Date:   Wed, 8 Apr 2020 15:23:14 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched/deadline: Improve admission control for
 asymmetric CPU capacities

On 04/08/20 15:30, luca abeni wrote:
> Hi Valentin,
> 
> On Wed, 08 Apr 2020 11:42:14 +0100
> Valentin Schneider <valentin.schneider@....com> wrote:
> 
> > On 08/04/20 10:50, Dietmar Eggemann wrote:
> > > +++ b/kernel/sched/sched.h
> > > @@ -304,11 +304,14 @@ void __dl_add(struct dl_bw *dl_b, u64 tsk_bw,
> > > int cpus) __dl_update(dl_b, -((s32)tsk_bw / cpus));
> > >  }
> > >
> > > +static inline unsigned long rd_capacity(int cpu);
> > > +
> > >  static inline
> > > -bool __dl_overflow(struct dl_bw *dl_b, int cpus, u64 old_bw, u64
> > > new_bw) +bool __dl_overflow(struct dl_bw *dl_b, int cpu, u64
> > > old_bw, u64 new_bw) {
> > >       return dl_b->bw != -1 &&
> > > -	       dl_b->bw * cpus < dl_b->total_bw - old_bw + new_bw;
> > > +	       cap_scale(dl_b->bw, rd_capacity(cpu)) <
> > > +	       dl_b->total_bw - old_bw + new_bw;
> > >  }
> > >  
> > 
> > I don't think this is strictly equivalent to what we have now for the
> > SMP case. 'cpus' used to come from dl_bw_cpus(), which is an ugly way
> > of writing
> > 
> >      cpumask_weight(rd->span AND cpu_active_mask);
> > 
> > The rd->cpu_capacity_orig field you added gets set once per domain
> > rebuild, so it also happens in sched_cpu_(de)activate() but is
> > separate from touching cpu_active_mask. AFAICT this mean we can
> > observe a CPU as !active but still see its capacity_orig accounted in
> > a root_domain.
> 
> Sorry, I suspect this is my fault, because the bug comes from my
> original patch.
> When I wrote the original code, I believed that when a CPU is
> deactivated it is also removed from its root domain.
> 
> I now see that I was wrong.

Shouldn't rd->online be equivalent to (rd->span & cpu-active_mask)?

--
Qais Yousef