linux-kernel - Re: [PATCH] sched/topology: Disable sched_asym

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Date:   Tue, 15 Oct 2019 10:47:49 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Quentin Perret <qperret@...gle.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...rret.net>,
        "# v4 . 16+" <stable@...r.kernel.org>
Subject: Re: [PATCH] sched/topology: Disable sched_asym_cpucapacity on domain
 destruction

On 15/10/2019 10:22, Dietmar Eggemann wrote:
> I still don't understand the benefit of the counter approach here.
> sched_smt_present counts the number of cores with SMT. So in case you
> have 2 SMT cores with 2 HW threads and you CPU hp out one CPU, you still
> have sched_smt_present, although 1 CPU doesn't have a SMT thread sibling
> anymore.
> 
> Valentin's patch makes sure that sched_asym_cpucapacity is correctly set
> when the sd hierarchy is rebuild due to CPU hp. Including the unlikely
> scenario that an asymmetric CPU capacity system (based on DT's
> capacity-dmips-mhz values) turns normal SMT because of the max frequency
> values of the CPUs involved.
> 
> Systems with a mix of asymmetric and symmetric CPU capacity rd's have to
> live with the fact that wake_cap and misfit handling is enabled for
> them. This should be the case already today.
> 

Good point, that's what I slowly came to realize this morning.

> There should be no SD_ASYM_CPUCAPACITY flag on the sd's of the CPUs of
> the symmetric CPU capacity rd's. I.e. update_top_cache_domain() should
> set sd_asym_cpucapacity=NULL for those CPUs.
> 
> So as a rule we could say even if a static key enables a code path, a
> derefenced sd still has to be checked against NULL.
> 

Yeah, I think there's no escaping it. Let me see if I can do something
sensible regarding the static key.