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

From: John Mathew <john.mathew@unikie.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	corbet@lwn.net, mingo@redhat.com, peterz@infradead.org,
	juri.lelli@redhat.com,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	rostedt@goodmis.org, Benjamin Segall <bsegall@google.com>,
	mgorman@suse.de, bristot@redhat.com, tsbogend@alpha.franken.de,
	Lukas Bulwahn <lukas.bulwahn@gmail.com>,
	x86@kernel.org, linux-mips@vger.kernel.org, tglx@linutronix.de,
	mostafa.chamanara@gmail.com, willy@infradead.org,
	Mostafa Chamanara <mostafa.chamanara@basemark.com>,
	Oleg Tsymbal <oleg.tsymbal@unikie.com>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [RFC PATCH v4 2/3] docs: scheduler: Add scheduler overview documentation
Date: Thu, 14 May 2020 12:24:49 +0300	[thread overview]
Message-ID: <CAJz2qXn+z3va-NuqXLfdgaG6Ah3S_f2SijmL7Jn96F-8SW_ndw@mail.gmail.com> (raw)
In-Reply-To: <jhjy2pwq6e2.mognet@arm.com>

On Wed, May 13, 2020 at 5:03 PM Valentin Schneider
<valentin.schneider@arm.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