sched/fair: Revert 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing...

sched/fair: Revert 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")

Zicheng Qu reported that, because avg_vruntime() always includes
cfs_rq->curr, when ->on_rq, place_entity() doesn't work right.

Specifically, the lag scaling in place_entity() relies on
avg_vruntime() being the state *before* placement of the new entity.
However in this case avg_vruntime() will actually already include the
entity, which breaks things.

Also, Zicheng Qu argues that avg_vruntime should be invariant under
reweight. IOW commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") was wrong!

The issue reported in 6d71a9c61604 could possibly be explained by
rounding artifacts -- notably the extreme weight '2' is outside of the
range of avg_vruntime/sum_w_vruntime, since that uses
scale_load_down(). By scaling vruntime by the real weight, but
accounting it in vruntime with a factor 1024 more, the average moves
significantly. However, that is now cured.

Tested by reverting 66951e4860d3 ("sched/fair: Fix update_cfs_group()
vs DELAY_DEQUEUE") and tracing vruntime and vlag figures again.

Reported-by: Zicheng Qu <quzicheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
Link: https://patch.msgid.link/20260219080625.066102672%40infradead.org