Date:   Tue, 18 May 2021 16:53:54 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Beata Michalska <beata.michalska@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        corbet@....net, rdunlap@...radead.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 2/3] sched/topology: Rework CPU capacity asymmetry detection

On 18/05/21 15:40, Beata Michalska wrote:
> On Mon, May 17, 2021 at 04:06:05PM +0100, Valentin Schneider wrote:
>> On 17/05/21 14:18, Beata Michalska wrote:
>> > On Mon, May 17, 2021 at 01:04:25PM +0100, Valentin Schneider wrote:
>> >> On 17/05/21 09:23, Beata Michalska wrote:
>> >> > +static void asym_cpu_capacity_scan(const struct cpumask *cpu_map)
>> >> > +{
>> >> > +	struct asym_cap_data *entry, *next;
>> >> > +	int cpu;
>> >> >
>> >> > -		for_each_sd_topology(tl) {
>> >> > -			if (tl_id < asym_level)
>> >> > -				goto next_level;
>> >> > +	if (!list_empty(&asym_cap_list))
>> >> > +		list_for_each_entry(entry, &asym_cap_list, link)
>> >> > +			cpumask_clear(entry->cpu_mask);
>> >> >
>> >>
>> >> The topology isn't going to change between domain rebuilds, so why
>> >> recompute the masks? The sched_domain spans are already masked by cpu_map,
>> >> so no need to do this masking twice. I'm thinking this scan should be done
>> >> once against the cpu_possible_mask - kinda like sched_init_numa() done once
>> >> against the possible nodes.
>> >>
>> > This is currently done, as what you have mentioned earlier, the tl->mask
>> > may contain CPUs that are not 'available'. So it makes sure that the masks
>> > kept on  the list are representing only those CPUs that are online.
>> > And it is also needed case all CPUs of given capacity go offline - not to to
>> > lose the full asymmetry that might change because of that ( empty masks are
>> > being removed from the list).
>> >
>> > I could change that and use the CPU mask that represents the online CPUs as
>> > a checkpoint but then it also means additional tracking which items on the
>> > list are actually available at a given point of time.
>> > So if the CPUs masks on the list are to be set once (as you are suggesting)
>> > than it needs additional logic to count the number of available capacities
>> > to decide whether there is a full asymmetry or not.
>> >
>>
>> That should be doable by counting non-empty intersections between each
>> entry->cpumask and the cpu_online_mask in _classify().
>>
>> That said I'm afraid cpufreq module loading forces us to dynamically update
>> those masks, as you've done. The first domain build could see asymmetry
>> without cpufreq loaded, and a later one with cpufreq loaded would need an
>> update. Conversely, as much of a fringe case as it is, we'd have to cope
>> with the cpufreq module being unloaded later on...
>>
>> :(
> So it got me thinking that maybe we could actually make it more
> 'update-on-demand' and use the cpufreq policy notifier to trigger the update.
> I could try to draft smth generic enough to make it ... relatively easy to adapt
> to different archs case needed.
> Any thoughts ?
>

The cpufreq policy notifier rebuild is currently an arch_topology.c
specificity, and perhaps we can consider this as our standing policy: if an
arch needs a topology rebuild upon X event (which isn't hotplug), it is
responsible for triggering it itself.

There's those sched_energy_update / arch_update_cpu_topology() bools that
are used to tweak the rebuild behaviour, perhaps you could gate the
capacity maps rebuild behind arch_update_cpu_topology()?

That way you could build those maps based on a cpu_possible_mask iterator,
and only rebuild them when the arch requests it (arch_topology already does
that with the cpufreq notifier). How does it sound?

> ---
> BR
> B.

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