Re: Cache-aware scheduling does not work well with amd big/little cores

[Klaus Kusche <klaus.kusche@computerix.info>]

Hello,

On 31/08/2026 19:29, Tim Chen wrote:
> On Mon, 2026-08-31 at 13:24 +0200, Klaus Kusche wrote:
>> Hello,
>>
>> both patches in combination seem to have the desired effect.
>>
>> But I just look at a bar graph showing the current load
>> of each core. 
>> The graph suggests that long-running CPU-intensive processes 
>> migrate to fast cores when fast cores become available.
>> And I have the impression that LTO compilations
>> finish significantly faster now.
>>
>> I don't have exact numbers or benchmarks.
> 
> Thanks for testing the fix.
> 
> If you just apply https://lore.kernel.org/lkml/20260825174112.2580942-1-tim.c.chen@linux.intel.com/,
> with default aggr_tolerance, what numbers do you see?
> 
> That will be helpful for further tuning.  Thanks.
> 
> Tim 

I did some quick testing (no perfect benchmark environment,
just checking runtime and CPU consumption with "time").

I timed a kernel build (-j 24 and full lto, my own .config)
and an application build (also with a lot of parallelism)
with three different kernels:

a) Cache aware scheduling completely configured off

b) Cache aware scheduling turned on, but without patch

c) Cache aware scheduling turned on, with patch

Big/little scheduling was always on, 
Mario's patch was always applied
(without it, results are significantly worse,
because I use kernels without debugfs,
so big/little scheduling is off without the patch).

Results:

There is no significant difference between b) and c) 
(<= 1 % wallclock time)
Sometimes b) is better, sometimes c) is better,
I'd say the differences are below the accuracy of my tests.

But a) was reproducibly better than b) and c)
w.r.t. wallclock time: 2-2.6 %
It was also very slightly better w.r.t. total kernel CPU seconds.
The results w.r.t. total usermode CPU seconds varied too much.
(I always ran the application build twice,
and for all a), b) and c), the second run consumed
significantly more usermode CPU seconds,
but took a little less wallclock time - I don't know why).

So in short, at least for the two build benchmarks I made,
and with respect to the wallclock time they took, AMD Ryzen HX 370 
does slightly better completely *without* cache aware scheduling,
but cache aware scheduling looses less than 3 %,
both with and without the patch.

The big difference I observed when 7.2 came out 
was perhaps due to the fact
that the older version of Mario's patch I had
did not apply correctly or did not work as expected with 7.2.

Greetings

-- 
Prof. Dr. Klaus Kusche
Privat: Söllmnitz 32 d, D-07554 Gera/Söllmnitz
036695/859909 klaus.kusche@computerix.info https://www.computerix.info
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klaus.kusche@dhge.de https://www.dhge.de