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

[John Mathew]

On Wed, May 13, 2020 at 5:03 PM Valentin Schneider
<valentin.schneider@xxxxxxx> 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 Âarch and frequencies), i.e. we need to
> rebuild the SD's again once cpufreq comes up.
Fixed in v5 of the patchset.
Thanks