EEVDF regression still exists

[Peter Zijlstra]

On Tue, Apr 29, 2025 at 04:38:17PM -0500, Cristian Prundeanu wrote:

> [1] https://github.com/aws/repro-collection/blob/main/repros/repro-mysql-EEVDF-regression/results/20250428/README.md

That 'perf sched stats diff' output is completely broken -- probably
trying to diff two different schedstat versions isn't working.

Anyway, looking at the two individual reports side by side:

 - schedule() left the processor idle             -- is up

vs.

 - pull_task() count on cpu newly idle            -- is down
 - load_balance() success count on cpu newly idle -- is down

Which seem related and would suggest we look at newidle balance. One of
the things we've seen before is that newidle was affected by the shorter
slice of EEVDF. But it is also quite possible something changed in the
load-balancer here.

Also of note is that .15 seems to have a lower number of 'ttwu() was
called to wake up on the local cpu' -- which I'm not quite sure how to
rhyme with the previous observation. The newidle thing seems to suggest
not enough migrations, while this would suggest too many migrations.