Re: [RFC][RFT][PATCH 0/3] arm64: Enable asympacking for minor CPPC asymmetry

[Andrea Righi <arighi@nvidia.com>]

Hi Christian,

On Wed, Mar 25, 2026 at 06:13:11PM +0000, Christian Loehle wrote:
...
> RFT:
> Andrea, please give this a try. This should perform better in particular
> for single-threaded workloads and workloads that do not utilize all
> cores (all the time anyway).
> Capacity-aware scheduling wakeup works very different to the SMP path
> used now, some workloads will benefit, some regress, it would be nice
> to get some test results for these.
> We already discussed DCPerf MediaWiki seems to benefit from
> capacity-aware scheduling wakeup behavior, but others (most?) should
> benefit from this series.
> 
> I don't know if we can also be clever about ordering amongst SMT siblings.
> That would be dependent on the uarch and I don't have a platform to
> experiment with this though, so consider this series orthogonal to the
> idle-core SMT considerations.
> On platforms with SMT though asympacking makes a lot more sense than
> capacity-aware scheduling, because arguing about capacity without
> considering utilization of the sibling(s) (and the resulting potential
> 'stolen' capacity we perceive) isn't theoretically sound.

I did some early testing with this patch set. On Vera I'm getting much
better performance that SD_ASYM_CPUCAPACITY of course (~1.5x avg speedup),
mostly because we avoid using both SMT siblings. It's still not the same
improvement that I get equalizing the capacity using the 5% threshold
(~1.8x speedup).

Of course I need to test with more workloads and I haven't tested it on
Grace yet, to check if we're regressing something, but in general it seems
functional.

Now it depends if SD_ASYM_PACKING is the route we want to take or if we
should start addressing SMT in SD_ASYM_CPUCAPACITY, as pointed by Vincent.
In general I think I agree with Vincent, independently on this particular
case, it'd be nice to start improving SD_ASYM_CPUCAPACITY to support SMT.

Thanks,
-Andrea