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

Beata Michalska
2021-05-26T21:40:25Z

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:
>
> [...]
>
> >>> BTW, how would this mechanism behave on a system with SMT and asymmetric CPU
> >>> capacity? Something EAS wouldn't allow but I guess asym_cap_list will be
> >>> constructed and the SD_ASYM_CPUCAPACITY_XXX flags will be set?
> >> Yes, the list would get created and flags set. I do not think there is
> >> a difference with current approach (?). So EAS would be disabled (it only cares
> >> about SD_ASYM_CPUCAPACITY_FULL flag) but the misift might still kick in.
> >>
> > That depends on the arch_scale_cpu_capacity. I would imagine it would
> > return SCHED_CAPACITY_SCALE for those, which means no asymmetry will
> > be detected ?
>
> I was thinking about an erroneous dts file like:
>
> 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 ?

---
BR
B.