BACKPORT: sched/fair: Fix EEVDF entity placement bug causing scheduling lag

I noticed this in my traces today:

       turbostat-1222    [006] d..2.   311.935649: reweight_entity: (ffff888108f13e00-ffff88885ef38440-6)
                               { weight: 1048576 avg_vruntime: 3184159639071 vruntime: 3184159640194 (-1123) deadline: 3184162621107 } ->
                               { weight: 2 avg_vruntime: 3184177463330 vruntime: 3184748414495 (-570951165) deadline: 4747605329439 }
       turbostat-1222    [006] d..2.   311.935651: reweight_entity: (ffff888108f13e00-ffff88885ef38440-6)
                               { weight: 2 avg_vruntime: 3184177463330 vruntime: 3184748414495 (-570951165) deadline: 4747605329439 } ->
                               { weight: 1048576 avg_vruntime: 3184176414812 vruntime: 3184177464419 (-1049607) deadline: 3184180445332 }

Which is a weight transition: 1048576 -> 2 -> 1048576.

One would expect the lag to shoot out *AND* come back, notably:

  -1123*1048576/2 = -588775424
  -588775424*2/1048576 = -1123

Except the trace shows it is all off. Worse, subsequent cycles shoot it
out further and further.

This made me have a very hard look at reweight_entity(), and
specifically the ->on_rq case, which is more prominent with
DELAY_DEQUEUE.

And indeed, it is all sorts of broken. While the computation of the new
lag is correct, the computation for the new vruntime, using the new lag
is broken for it does not consider the logic set out in place_entity().

With the below patch, I now see things like:

    migration/12-55      [012] d..3.   309.006650: reweight_entity: (ffff8881e0e6f600-ffff88885f235f40-12)
                               { weight: 977582 avg_vruntime: 4860513347366 vruntime: 4860513347908 (-542) deadline: 4860516552475 } ->
                               { weight: 2 avg_vruntime: 4860528915984 vruntime: 4860793840706 (-264924722) deadline: 6427157349203 }
    migration/14-62      [014] d..3.   309.006698: reweight_entity: (ffff8881e0e6cc00-ffff88885f3b5f40-15)
                               { weight: 2 avg_vruntime: 4874472992283 vruntime: 4939833828823 (-65360836540) deadline: 6316614641111 } ->
                               { weight: 967149 avg_vruntime: 4874217684324 vruntime: 4874217688559 (-4235) deadline: 4874220535650 }

Which isn't perfect yet, but much closer.

Bug: 438500731
Reported-by: Doug Smythies <dsmythies@telus.net>
Reported-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Change-Id: I33d54fb4e09a607779aa3130d831eadf09106518
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: eab03c23 ("sched/eevdf: Fix vruntime adjustment on reweight")
Link: https://lore.kernel.org/r/20250109105959.GA2981@noisy.programming.kicks-ass.net


[Trivial conflict in fair.c::reweight_entity() due to vendor hook around
changed code.]
(cherry picked from commit 6d71a9c6)
Signed-off-by: Qais Yousef <qyousef@google.com>