Date:   Tue, 25 May 2021 00:19:44 +0100
From:   Beata Michalska <beata.michalska@....com>
To:     Valentin Schneider <valentin.schneider@....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 v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection

On Mon, May 24, 2021 at 11:55:08PM +0100, Beata Michalska wrote:
> On Mon, May 24, 2021 at 07:01:04PM +0100, Valentin Schneider wrote:
> > Hi Beata,
> > 
> > On 24/05/21 11:16, Beata Michalska wrote:
> > > Currently the CPU capacity asymmetry detection, performed through
> > > asym_cpu_capacity_level, tries to identify the lowest topology level
> > > at which the highest CPU capacity is being observed, not necessarily
> > > finding the level at which all possible capacity values are visible
> > > to all CPUs, which might be bit problematic for some possible/valid
> > > asymmetric topologies i.e.:
> > >
> > > DIE      [                                ]
> > > MC       [                       ][       ]
> > >
> > > CPU       [0] [1] [2] [3] [4] [5]  [6] [7]
> > > Capacity  |.....| |.....| |.....|  |.....|
> > >            L	     M       B        B
> > >
> > > Where:
> > >  arch_scale_cpu_capacity(L) = 512
> > >  arch_scale_cpu_capacity(M) = 871
> > >  arch_scale_cpu_capacity(B) = 1024
> > >
> > > In this particular case, the asymmetric topology level will point
> > > at MC, as all possible CPU masks for that level do cover the CPU
> > > with the highest capacity. It will work just fine for the first
> > > cluster, not so much for the second one though (consider the
> > > find_energy_efficient_cpu which might end up attempting the energy
> > > aware wake-up for a domain that does not see any asymmetry at all)
> > >
> > > Rework the way the capacity asymmetry levels are being detected,
> > > allowing to point to the lowest topology level (for a given CPU), where
> > > full set of available CPU capacities is visible to all CPUs within given
> > > domain. As a result, the per-cpu sd_asym_cpucapacity might differ across
> > > the domains. This will have an impact on EAS wake-up placement in a way
> > > that it might see different rage of CPUs to be considered, depending on
> > > the given current and target CPUs.
> > >
> > > Additionally, those levels, where any range of asymmetry (not
> > > necessarily full) is being detected will get identified as well.
> > > The selected asymmetric topology level will be denoted by
> > > SD_ASYM_CPUCAPACITY_FULL sched domain flag whereas the 'sub-levels'
> > > would receive the already used SD_ASYM_CPUCAPACITY flag. This allows
> > > maintaining the current behaviour for asymmetric topologies, with
> > > misfit migration operating correctly on lower levels, if applicable,
> > > as any asymmetry is enough to trigger the misfit migration.
> > > The logic there relies on the SD_ASYM_CPUCAPACITY flag and does not
> > > relate to the full asymmetry level denoted by the sd_asym_cpucapacity
> > > pointer.
> > >
> > > Detecting the CPU capacity asymmetry is being based on a set of
> > > available CPU capacities for all possible CPUs. This data is being
> > > generated upon init and updated once CPU topology changes are being
> > > detected (through arch_update_cpu_topology). As such, any changes
> > > to identified CPU capacities (like initializing cpufreq) need to be
> > > explicitly advertised by corresponding archs to trigger rebuilding
> > > the data.
> > >
> > > This patch also removes the additional -dflags- parameter used when
> >   ^^^^^^^^^^^^^^^^^^^^^^^
> > s/^/Also remove/
> I would kind of ... disagree.
> All the commit msg is constructed using passive structure, the suggestion
> would actually break that. And it does 'sound' bit imperative but I guess
> that is subjective. I'd rather stay with impersonal structure (which is
> applied through out the whole patchset).
> > 
> > > building sched domains as the asymmetry flags are now being set
> > > directly in sd_init.
> > >
> > 
> > Few nits below, but beyond that:
> > 
> > Tested-by: Valentin Schneider <valentin.schneider@....com>
> > Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> > 
> Thanks a lot for the review and testing!
> 
> > > +static inline int
> > > +asym_cpu_capacity_classify(struct sched_domain *sd,
> > > +			   const struct cpumask *cpu_map)
> > > +{
> > > +	int sd_asym_flags = SD_ASYM_CPUCAPACITY | SD_ASYM_CPUCAPACITY_FULL;
> > > +	struct asym_cap_data *entry;
> > > +	int asym_cap_count = 0;
> > > +
> > > +	if (list_is_singular(&asym_cap_list))
> > > +		goto leave;
> > > +
> > > +	list_for_each_entry(entry, &asym_cap_list, link) {
> > > +		if (cpumask_intersects(sched_domain_span(sd), entry->cpu_mask)) {
> > > +			++asym_cap_count;
> > > +		} else {
> > > +			/*
> > > +			 * CPUs with given capacity might be offline
> > > +			 * so make sure this is not the case
> > > +			 */
> > > +			if (cpumask_intersects(entry->cpu_mask, cpu_map)) {
> > > +				sd_asym_flags &= ~SD_ASYM_CPUCAPACITY_FULL;
> > > +				if (asym_cap_count > 1)
> > > +					break;
> > > +			}
> > 
> > Readability nit: That could be made into an else if ().
> It could but then this way the -comment- gets more exposed.
> But that might be my personal perception so I can change that.
> > 
> > 
> > > +		}
> > > +	}
> > > +	WARN_ON_ONCE(!asym_cap_count);
> > > +leave:
> > > +	return asym_cap_count > 1 ? sd_asym_flags : 0;
> > > +}
> > > +
> > 
> > > +static void asym_cpu_capacity_scan(void)
> > > +{
> > > +	struct asym_cap_data *entry, *next;
> > > +	int cpu;
> > > +
> > > +	list_for_each_entry(entry, &asym_cap_list, link)
> > > +		cpumask_clear(entry->cpu_mask);
> > > +
> > > +	entry = list_first_entry_or_null(&asym_cap_list,
> > > +					 struct asym_cap_data, link);
> > > +
> > > +	for_each_cpu_and(cpu, cpu_possible_mask,
> > > +			 housekeeping_cpumask(HK_FLAG_DOMAIN)) {
> > > +		unsigned long capacity = arch_scale_cpu_capacity(cpu);
> > > +
> > > +		if (!entry || capacity != entry->capacity)
> > > +			entry = asym_cpu_capacity_get_data(capacity);
> > > +		if (entry)
> > > +			__cpumask_set_cpu(cpu, entry->cpu_mask);
> > 
> > That 'if' is only there in case the alloc within the helper failed, which
> > is a bit of a shame.
> > 
> > You could pass the CPU to that helper function and have it set the right
> > bit, or you could even forgo the capacity != entry->capacity check here and
> > let the helper function do it all.
> > 
> > Yes, that means more asym_cap_list iterations, but that's
> > O(nr_cpus * nr_caps); a topology rebuild is along the lines of
> > O(nr_cpus² * nr_topology_levels), so not such a big deal comparatively.
> > 
> I could drop that check and make the helper function update the CPUs mask
> (along with dropping the initial grabbing of the first entry)
> +
> switching to list_for_each_entry_reverse which would result in less
> iterations for most (if not all) of the use cases.
> 
Ignore the 'reverse' idea - the items are already prepended so regular
iteration should pick the last item added.

---
BR
B.

> 
> ---
> BR
> B
> > > +	}
> > > +
> > > +	list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
> > > +		if (cpumask_empty(entry->cpu_mask)) {
> > > +			list_del(&entry->link);
> > > +			kfree(entry);
> > > +		}
> > > +	}
> > > +}
> > > +

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