sched/fair: Fix zero_vruntime tracking

sched/fair: Fix zero_vruntime tracking

It turns out that zero_vruntime tracking is broken when there is but a single
task running. Current update paths are through __{en,de}queue_entity(), and
when there is but a single task, pick_next_task() will always return that one
task, and put_prev_set_next_task() will end up in neither function.

This can cause entity_key() to grow indefinitely large and cause overflows,
leading to much pain and suffering.

Furtermore, doing update_zero_vruntime() from __{de,en}queue_entity(), which
are called from {set_next,put_prev}_entity() has problems because:

 - set_next_entity() calls __dequeue_entity() before it does cfs_rq->curr = se.
   This means the avg_vruntime() will see the removal but not current, missing
   the entity for accounting.

 - put_prev_entity() calls __enqueue_entity() before it does cfs_rq->curr =
   NULL. This means the avg_vruntime() will see the addition *and* current,
   leading to double accounting.

Both cases are incorrect/inconsistent.

Noting that avg_vruntime is already called on each {en,de}queue, remove the
explicit avg_vruntime() calls (which removes an extra 64bit division for each
{en,de}queue) and have avg_vruntime() update zero_vruntime itself.

Additionally, have the tick call avg_vruntime() -- discarding the result, but
for the side-effect of updating zero_vruntime.

While there, optimize avg_vruntime() by noting that the average of one value is
rather trivial to compute.

Test case:
  # taskset -c -p 1 $$
  # taskset -c 2 bash -c 'while :; do :; done&'
  # cat /sys/kernel/debug/sched/debug | awk '/^cpu#/ {P=0} /^cpu#2,/ {P=1} {if (P) print $0}' | grep -e zero_vruntime -e "^>"

PRE:
    .zero_vruntime                 : 31316.407903
  >R            bash   487     50787.345112   E       50789.145972           2.800000     50780.298364        16     120         0.000000         0.000000         0.000000        /
    .zero_vruntime                 : 382548.253179
  >R            bash   487    427275.204288   E      427276.003584           2.800000    427268.157540        23     120         0.000000         0.000000         0.000000        /

POST:
    .zero_vruntime                 : 17259.709467
  >R            bash   526     17259.709467   E       17262.509467           2.800000     16915.031624         9     120         0.000000         0.000000         0.000000        /
    .zero_vruntime                 : 18702.723356
  >R            bash   526     18702.723356   E       18705.523356           2.800000     18358.045513         9     120         0.000000         0.000000         0.000000        /

Fixes: 79f3f9bedd14 ("sched/eevdf: Fix min_vruntime vs avg_vruntime")
Reported-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
Link: https://patch.msgid.link/20260219080624.438854780%40infradead.org