linux-kernel - Re: [PATCH 1/4] sched/topology: SD_ASYM

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Message-ID: <20180724083710.GB29978@e105550-lin.cambridge.arm.com>
Date:   Tue, 24 Jul 2018 09:37:10 +0100
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     vincent.guittot@...aro.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        mingo@...hat.com, valentin.schneider@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] sched/topology: SD_ASYM_CPUCAPACITY flag detection

On Mon, Jul 23, 2018 at 05:07:50PM +0100, Qais Yousef wrote:
> On 23/07/18 16:27, Morten Rasmussen wrote:
> >It does increase the cost of things like hotplug slightly and
> >repartitioning of root_domains a slightly but I don't see how we can
> >avoid it if we want generic code to set this flag. If the costs are not
> >acceptable I think the only option is to make the detection architecture
> >specific.
> 
> I think hotplug is already expensive and this overhead would be small in
> comparison. But this could be called when frequency changes if I understood
> correctly - this is the one I wasn't sure how 'hot' it could be. I wouldn't
> expect frequency changes at a very high rate because it's relatively
> expensive too..

A frequency change shouldn't lead to a flag change or a rebuild of the
sched_domain hierarhcy. The situations where the hierarchy should be
rebuild to update the flag is during boot as we only know the amount of
asymmetry once cpufreq has been initialized, when cpus are hotplugged
in/out, and when root_domains change due to cpuset reconfiguration. So
it should be a relatively rare event.