linux-kernel - Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry detection

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Message-ID: <4f43a9a8-b64e-bb47-b3c1-f51165f40249@arm.com>
Date:   Wed, 2 Jun 2021 19:17:12 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Beata Michalska <beata.michalska@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, corbet@....net, rdunlap@...radead.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 2/3] sched/topology: Rework CPU capacity asymmetry
 detection

On 27/05/2021 19:07, Beata Michalska wrote:
> On Thu, May 27, 2021 at 05:08:42PM +0200, Dietmar Eggemann wrote:
>> On 26/05/2021 23:40, Beata Michalska wrote:
>>> On Wed, May 26, 2021 at 08:17:41PM +0200, Dietmar Eggemann wrote:
>>>> On 26/05/2021 14:51, Beata Michalska wrote:
>>>>> On Wed, May 26, 2021 at 01:15:46PM +0100, Beata Michalska wrote:
>>>>>> On Wed, May 26, 2021 at 11:52:25AM +0200, Dietmar Eggemann wrote:
>>>>>>> On 25/05/2021 12:29, Beata Michalska wrote:
>>>>>>>> On Tue, May 25, 2021 at 10:53:07AM +0100, Valentin Schneider wrote:
>>>>>>>>> On 24/05/21 23:55, Beata Michalska wrote:
>>>>>>>>>> On Mon, May 24, 2021 at 07:01:04PM +0100, Valentin Schneider wrote:
>>>>>>>>>>> On 24/05/21 11:16, Beata Michalska wrote:

[...]

>>> We could possibly add a warning (like in EAS) if the asymmetry is detected
>>> for SMT which would give some indication that there is smth ... wrong ?
>>
>> Maybe, in case you find an easy way to detect this.
>>
>> But the issue already exists today. Not with the topology mentioned
>> above but in case we slightly change it to:
>>
>>   cpus = { ([446 1024] [871 1024] [446 1024] ) ([1024 1024]) }
>>                                        ^^^^
>> so that we have a 1024 CPU in the lowest sd for each CPU, we would get
>> SD_ASYM_CPUCAPACITY on SMT.
> The asymmetry capacity flags are being set on a sched domain level, so
> we could use the SD_SHARE_CPUCAPACITY|SD_SHARE_PKG_RESOURCES (cpu_smt_flags)
> flags to determine if having asymmetry is valid or not ? If this is enough 
> this could be handled by the classify function?

Or maybe something directly in sd_init(), like the WARN_ONCE() which triggers
if somebody wants to sneak in a ~topology flag via a
sched_domain_topology_level table? 

IMHO checking `SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY` will be sufficient
here.

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 62d412013df8..77b73abbb9a4 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1561,6 +1561,11 @@ sd_init(struct sched_domain_topology_level *tl,
        sd_id = cpumask_first(sched_domain_span(sd));
 
        sd->flags |= asym_cpu_capacity_classify(sd, cpu_map);
+
+       WARN_ONCE((sd->flags & (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY)) ==
+                 (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY),
+                     "CPU capacity asymmetry not supported on SMT\n");
+
        /*
         * Convert topological properties into behaviour.
         */

In case we can agree on something simple here I guess you can incorporate it into v7.