sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task

sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task

CPUs whose rq only have SCHED_IDLE tasks running are considered to be
equivalent to truly idle CPUs during wakeup path. For fork and exec
SCHED_IDLE is even preferred.
This is based on the assumption that the SCHED_IDLE CPU is not in an
idle state and might be in a higher P-state, allowing the task/wakee
to run immediately without sharing the rq.

However this assumption doesn't hold if the wakee has SCHED_IDLE policy
itself, as it will share the rq with existing SCHED_IDLE tasks. In this
case, we are better off continuing to look for a truly idle CPU.

On a Intel Xeon 2-socket with 64 logical cores in total this yields
for kernel compilation using SCHED_IDLE:

+---------+----------------------+----------------------+--------+
| workers | mainline (seconds)   | patch (seconds)      | delta% |
+=========+======================+======================+========+
|       1 | 4384.728 ± 21.085    | 3843.250 ± 16.235    | -12.35 |
|       2 | 2242.513 ± 2.099     | 1971.696 ± 2.842     | -12.08 |
|       4 | 1199.324 ± 1.823     | 1033.744 ± 1.803     | -13.81 |
|       8 |  649.083 ± 1.959     |  559.123 ± 4.301     | -13.86 |
|      16 |  370.425 ± 0.915     |  325.906 ± 4.623     | -12.02 |
|      32 |  234.651 ± 2.255     |  217.266 ± 0.253     |  -7.41 |
|      64 |  202.286 ± 1.452     |  197.977 ± 2.275     |  -2.13 |
|     128 |  217.092 ± 1.687     |  212.164 ± 1.138     |  -2.27 |
+---------+----------------------+----------------------+--------+

Signed-off-by: Christian Loehle <christian.loehle@arm.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>
Link: https://patch.msgid.link/20260203184939.2138022-1-christian.loehle@arm.com