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

[Christian Loehle <christian.loehle@arm.com>]

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 160-CPU, dual-socket Altra has unusually large 80-CPU
candidate groups at NUMA level, making this particularly prone to stale
idle picks.

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

  --fork  --fork-max      Baseline       Patched    Change
  ------------------------------------------------------
       1           1        779.19        811.38    +4.13%
       8           1       2958.32       2960.18    +0.06%
      16           1       5070.95       5247.63    +3.48%
      16           4       7692.64       7831.60    +1.81%
      32           1       8662.82       8643.39    -0.22%
      64           1      11880.47      12096.69    +1.82%

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%       0.407%
fork, 64 creators          1.242%       0.672%

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 | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)


base-commit: fd73f4a6659897191fa0d40695fe370925dd3780
-- 
2.34.1