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

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Message-ID: <14593ba7-eed9-f035-724c-5cadbb859adc@arm.com>
Date:   Thu, 27 May 2021 17:08:42 +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 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:

[...]

>>                 cpu-map {
>>                         cluster0 {
>>                                 core0 {
>> 					thread0 {
>>                                         	cpu = <&A53_0>;
>> 					};
>> 					thread1 {
>>                                         	cpu = <&A53_1>;
>> 					};
>>                                 };
>>                                 core1 {
>> 					thread0 {
>>                                         	cpu = <&A53_2>;
>> 					};
>> 					thread1 {
>>                                         	cpu = <&A53_3>;
>> 					};
>>                                 };
>>                                 core2 {
>> 					thread0 {
>>                                         	cpu = <&A53_4>;
>> 					};
>> 					thread1 {
>>                                         	cpu = <&A53_5>;
>> 					};
>>                                 };
>>                         };
>>
>>                         cluster1 {
>>                                 core0 {
>> 					thread0 {
>>                                         	cpu = <&A53_6>;
>> 					};
>> 					thread1 {
>>                                         	cpu = <&A53_7>;
>> 					};
>>                                 };
>>                         };
>>                 };
>>
>> 		A53_0: cpu@0 {
>> 			capacity-dmips-mhz = <446>;
>> 	 	A53_1: cpu@1 {
>> 			capacity-dmips-mhz = <1024>;
>> 		A53_2: cpu@2 {
>> 			capacity-dmips-mhz = <871>;
>> 		A53_3: cpu@3 {
>> 			capacity-dmips-mhz = <1024>;
>> 		A53_4: cpu@4 {
>> 			capacity-dmips-mhz = <446>;
>> 		A53_5: cpu@5 {
>> 			capacity-dmips-mhz = <871>;
>> 		A53_6: cpu@6 {
>> 			capacity-dmips-mhz = <1024>;
>> 		A53_7: cpu@7 {
>> 			capacity-dmips-mhz = <1024>;
>>
>> Here I guess SD_ASYM_CPUCAPACITY will be attached to SMT[0-5]. So this
>> 'capacity-dmips-mhz' config error won't be detected.
>>
>> In case all CPUs (i.e. hw threads would have the correct
>> capacity-dmips-mhz = <1024> or not being set (default 1024))
>> asym_cap_list would corrcetly only have 1 entry.
> 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.