linux-kernel - Re: [PATCH v6 2/3] sched/topology: Rework CPU capacity asymmetry detection

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Date:   Wed, 02 Jun 2021 13:50:21 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Beata Michalska <beata.michalska@....com>,
        linux-kernel@...r.kernel.org
Cc:     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 v6 2/3] sched/topology: Rework CPU capacity asymmetry detection

On 27/05/21 16:38, Beata Michalska wrote:
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Suggested-by: Valentin Schneider <valentin.schneider@....com>
> Signed-off-by: Beata Michalska <beata.michalska@....com>

I ran this through the usual series of tests ('exotic' topologies, hotplug
and exclusive cpusets), it all behaves as expected.

Tested-by: Valentin Schneider <valentin.schneider@....com>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>

Some tiny cosmetic nits below, which don't warrant a new revision, and a
comment wrt purely symmetric systems.

> ---
>  kernel/sched/topology.c | 194 ++++++++++++++++++++++++----------------
>  1 file changed, 118 insertions(+), 76 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 55a0a243e871..77e6f79235ad 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c

> +/*
> + * Verify whether there is any CPU capacity asymmetry in a given sched domain.
> + * Provides sd_flags reflecting the asymmetry scope.
> + */
> +static inline int
> +asym_cpu_capacity_classify(struct sched_domain *sd,
> +			   const struct cpumask *cpu_map)
> +{
> +	struct asym_cap_data *entry;
> +	int sd_asym_flags = 0;
> +	int asym_cap_count = 0;
> +	int asym_cap_miss = 0;
> +
> +	/*
> +	 * Count how many unique CPU capacities this domain spans across
> +	 * (compare sched_domain CPUs mask with ones representing  available
> +	 * CPUs capacities). Take into account CPUs that might be offline:
> +	 * skip those.
> +	 */
> +	list_for_each_entry(entry, &asym_cap_list, link) {
> +		if (cpumask_intersects(sched_domain_span(sd),
> +				       cpu_capacity_span(entry)))

IMO this is one such place where the 80 chars limit can be omitted.

> +			++asym_cap_count;
> +		else if (cpumask_intersects(cpu_capacity_span(entry), cpu_map))
> +			++asym_cap_miss;
> +	}

> +/*
> + * Build-up/update list of CPUs grouped by their capacities
> + * An update requires explicit request to rebuild sched domains
> + * with state indicating CPU topology changes.
> + */
> +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(cpu_capacity_span(entry));
> +
> +	for_each_cpu_and(cpu, cpu_possible_mask,
> +			 housekeeping_cpumask(HK_FLAG_DOMAIN))

Ditto on keeping this on a single line.

> +		asym_cpu_capacity_update_data(cpu);
> +
> +	list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
> +		if (cpumask_empty(cpu_capacity_span(entry))) {
> +			list_del(&entry->link);
> +			kfree(entry);
> +		}
> +	}
> +}

One "corner case" that comes to mind is systems / architectures which are
purely symmetric wrt CPU capacity. Our x86 friends might object to us
reserving a puny 24 bytes + cpumask_size() in a corner of their
memory.

Perhaps we could clear the list in the list_is_singular_case(), and since
the rest of the code only does list iteration, this should 'naturally'
cover this case:

---
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 62d412013df8..b06d277fa280 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1305,14 +1305,13 @@ asym_cpu_capacity_classify(struct sched_domain *sd,
 	 * skip those.
 	 */
 	list_for_each_entry(entry, &asym_cap_list, link) {
-		if (cpumask_intersects(sched_domain_span(sd),
-				       cpu_capacity_span(entry)))
+		if (cpumask_intersects(sched_domain_span(sd), cpu_capacity_span(entry)))
 			++asym_cap_count;
 		else if (cpumask_intersects(cpu_capacity_span(entry), cpu_map))
 			++asym_cap_miss;
 	}
 	/* No asymmetry detected */
-	if (WARN_ON_ONCE(!asym_cap_count) || asym_cap_count == 1)
+	if (asym_cap_count < 2)
 		goto leave;
 
 	sd_asym_flags |= SD_ASYM_CPUCAPACITY;
@@ -1360,8 +1359,7 @@ static void asym_cpu_capacity_scan(void)
 	list_for_each_entry(entry, &asym_cap_list, link)
 		cpumask_clear(cpu_capacity_span(entry));
 
-	for_each_cpu_and(cpu, cpu_possible_mask,
-			 housekeeping_cpumask(HK_FLAG_DOMAIN))
+	for_each_cpu_and(cpu, cpu_possible_mask, housekeeping_cpumask(HK_FLAG_DOMAIN))
 		asym_cpu_capacity_update_data(cpu);
 
 	list_for_each_entry_safe(entry, next, &asym_cap_list, link) {
@@ -1370,6 +1368,16 @@ static void asym_cpu_capacity_scan(void)
 			kfree(entry);
 		}
 	}
+
+	/*
+	 * There's only one capacity value, i.e. this system is symmetric.
+	 * No need to keep this data around.
+	 */
+	if (list_is_singular(&asym_cap_list)) {
+		entry = list_first_entry(&asym_cap_list, typeof(*entry), link);
+		list_del(&entry->link);
+		kfree(entry);
+	}
 }
 
 /*