linux-kernel - Re: [PATCH v4 2/3] sched/topology: Rework CPU capacity asymmetry detection

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Date:   Mon, 17 May 2021 16:06:05 +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 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...

:(