linux-kernel - Re: [RFC PATCH v4 2/3] docs: scheduler: Add scheduler overview documentation

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Date:   Thu, 14 May 2020 12:24:49 +0300
From:   John Mathew <john.mathew@...kie.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        corbet@....net, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        rostedt@...dmis.org, Benjamin Segall <bsegall@...gle.com>,
        mgorman@...e.de, bristot@...hat.com, tsbogend@...ha.franken.de,
        Lukas Bulwahn <lukas.bulwahn@...il.com>, x86@...nel.org,
        linux-mips@...r.kernel.org, tglx@...utronix.de,
        mostafa.chamanara@...il.com, willy@...radead.org,
        Mostafa Chamanara <mostafa.chamanara@...emark.com>,
        Oleg Tsymbal <oleg.tsymbal@...kie.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC PATCH v4 2/3] docs: scheduler: Add scheduler overview documentation

On Wed, May 13, 2020 at 5:03 PM Valentin Schneider
<valentin.schneider@....com> wrote:
>
>
> On 13/05/20 14:43, john mathew wrote:
> > +=========================
> > +Capacity-Aware Scheduling
> > +=========================
> > +
>
> Thanks for taking a jab at this. At a glance it looks okay, with one
> comment below.
>
> FWIW I still intend to write a more pamphlet-sized thing, I'll toss
> something out in the coming weeks - depending on where this goes, I might
> base it on this.
>
> > +Scheduling load balancing on Asymmetric Multiprocessor systems was improved
> > +through the introduction of Capacity-Aware Scheduling. It identifies the
> > +most efficient CPU to assign a task based on its capacity. This capacity
> > +may be asymmetric due to heterogeneous computing architecture such
> > +as ARM big.LITTLE. Scheduler gets information about asymmetric capacities
> > +when the scheduler domain hierarchy is built using build_sched_domains().
> > +CPU capacities are provided to the scheduler topology code through the
> > +architecture specific implementation of the arch_scale_cpu_capacity().
> > +The SD_ASYM_CPUCAPACITY flag is set by the scheduler topology for a domain
> > +in the hierarchy where all CPU capacities are visible for any cpu's point
> > +of view on asymmetric CPU capacity systems. The scheduler can then take
> > +capacity asymmetry into account when load balancing.
> > +
> > +Initial CPU capacities are derived from the Device Tree and CPU frequency.
> > +For RISC-V & ARM64 it is done in drivers/base/arch_topology.c. A cpu-map
> > +device tree is parsed to obtain the cpu topology and the initial CPU capacity
> > +is set using the CPUFreq subsystem. A callback is registered to the CPUFreq
> > +subsystem to rebuild sched_domains when CPU frequency changes.
> > +
>
> We don't rebuild domains on frequency changes (that would be ludicrous!),
> rather we do that on policy changes. It's mostly because we need to wait
> for cpufreq to be loaded before having a complete view over the capacities
> of the CPUs (which is a mix of µarch and frequencies), i.e. we need to
> rebuild the SD's again once cpufreq comes up.
Fixed in v5 of the patchset.
Thanks