[PATCH v2 0/2] sched/fair: Randomize equally shallow idle CPU picks

[Christian Loehle]

Concurrent slow-path selectors can converge on the same idle CPU before
either task is enqueued. Remove the idle-recency preference and randomize
equal-latency choices in a single scan.

The testing platform is a 160-CPU, dual-socket Altra with unusually large
80-CPU candidate groups at NUMA level.

Median stress-ng throughput (bogo ops/s):

  --fork  --fork-max      Baseline       Patched    Change
  ------------------------------------------------------
       1           1        779.19        829.82    +6.50%
      16           1       5070.95       5277.62    +4.08%
      16           4       7692.64       7963.30    +3.52%
      32           1       8662.82       8733.79    +0.82%
      64           1      11880.47      12001.01    +1.01%

Separate instrumented runs observed lower conditional stale-pick rates,
i.e. a busy candidate at final return:

Workload                Baseline       Patched
----------------------------------------------
fork, 32 creators          0.771%       0.335%
fork, 64 creators          1.242%       0.633%

Changes since v1:
- Use u64 latency keys and U64_MAX for unpublished states.
- Sample unpublished-state CPUs only as fallbacks, resetting the reservoir
  when the first advertised-state candidate is found.
- Update the Altra measurements.
- Pick up Vincent Guittot's Reviewed-by for patch 1.

Christian Loehle (2):
  sched/fair: Drop idle recency from slow-path CPU selection
  sched/fair: Randomize equally shallow slow-path candidates

 kernel/sched/fair.c | 31 ++++++++++++-------------------
 1 file changed, 12 insertions(+), 19 deletions(-)


base-commit: fd73f4a6659897191fa0d40695fe370925dd3780
-- 
2.34.1