From: Valentin Schneider <valentin.schneider@arm.com>
To: Beata Michalska <beata.michalska@arm.com>, linux-kernel@vger.kernel.org
Cc: peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
corbet@lwn.net, rdunlap@infradead.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v6 2/3] sched/topology: Rework CPU capacity asymmetry detection
Date: Wed, 02 Jun 2021 13:50:21 +0100 [thread overview]
Message-ID: <87eedkfn1u.mognet@arm.com> (raw)
In-Reply-To: <20210527153842.17567-3-beata.michalska@arm.com>
On 27/05/21 16:38, Beata Michalska wrote:
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Beata Michalska <beata.michalska@arm.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@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.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);
+ }
}
/*
next prev parent reply other threads:[~2021-06-02 12:50 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-27 15:38 [PATCH v6 0/3] Rework CPU capacity asymmetry detection Beata Michalska
2021-05-27 15:38 ` [PATCH v6 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched_domain flag Beata Michalska
2021-05-27 15:38 ` [PATCH v6 2/3] sched/topology: Rework CPU capacity asymmetry detection Beata Michalska
2021-06-02 12:50 ` Valentin Schneider [this message]
2021-06-02 13:03 ` Beata Michalska
2021-06-02 19:09 ` Dietmar Eggemann
2021-06-02 19:54 ` Beata Michalska
2021-05-27 15:38 ` [PATCH v6 3/3] sched/doc: Update the CPU capacity asymmetry bits Beata Michalska
2021-06-02 19:10 ` [PATCH v6 0/3] Rework CPU capacity asymmetry detection Dietmar Eggemann
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