Re: [PATCH] sched/topology: Avoid spurious asymmetry from CPU capacity noise

[Dietmar Eggemann]

On 24.03.26 12:01, Andrea Righi wrote:
> Hi Dietmar,
> 
> On Tue, Mar 24, 2026 at 11:29:24AM +0100, Dietmar Eggemann wrote:
>> On 24.03.26 10:46, Andrea Righi wrote:
>>> Hi Christian,
>>>
>>> On Tue, Mar 24, 2026 at 08:08:22AM +0000, Christian Loehle wrote:
>>>> On 3/24/26 07:55, Christian Loehle wrote:
>>>>> On 3/24/26 07:39, Vincent Guittot wrote:
>>>>>> On Tue, 24 Mar 2026 at 01:55, Andrea Righi <arighi@xxxxxxxxxx> wrote:

[...]

>> The first time we observed this on NVIDIA Grace, we wondered whether
>> there might be functionality outside the task scheduler that makes use
>> of these slightly heterogeneous CPU capacity values from CPPC—and
>> whether the dependency on task scheduling was simply an overlooked
>> phenomenon.
>>
>> And then there was DCPerf Mediawiki on 72 CPUs system always scoring
>> better with sched_asym_cpucap_active() = TRUE (mentioned already by
>> Chris L. in:
>> https://lore.kernel.org/r/15ffdeb3-a0f3-4b88-92c0-17ffb03b0574@xxxxxxx
> 
> Yeah, I think Chris' asym-packing approach might be the safest thing to do.
> 
> At the same time it would be nice to improve asym-capacity to introduce
> some concept of SMT awareness, that was my original attempt with
> https://lore.kernel.org/all/20260318092214.130908-1-arighi@xxxxxxxxxx,
> since we may see similar asym-capacity benefits on Vera (that has SMT,
> unlike Grace). What do you think?

We never found a good way to specify a CPU capacity in the SMT case (EAS
and energy model included). So comparing CPU capacity w/ utilization, CPU
overutilization detection etc. definitions get more blurry.

But in case you now want to hide these small CPU capacity differences from
asym-cpucap setup you won't run into this 'SD_SHARE_CPUCAPACITY +
SD_ASYM_CPUCAPACITY'.

You still will have small differences in sched group capacities but this
is covered by load-balance.

BTW, you should have seen on Vera ?: 

sd_int() [kernel/sched/.topology.c]

1720 WARN_ONCE((sd->flags & (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY)) ==
1721                   (SD_SHARE_CPUCAPACITY | SD_ASYM_CPUCAPACITY),
1722                   "CPU capacity asymmetry not supported on SMT\n");