Re: [PATCH] timers: Clear pending_map on CPU hotplug

[Liang Hao <haohlliang@gmail.com>]

Hi,

Additional measurements on a local arm64 Android board.

Method (two sessions per build):
  5 runs per session, 5 offline/online pairs per run.

Per pair:
  taskset 0x8 sh -c 'n=0; while [ $n -lt 100 ]; do sleep 0.01; n=$((n+1)); done' &
  sleep 0.5
  echo 0 > /sys/devices/system/cpu/cpu3/online
  echo 1 > /sys/devices/system/cpu/cpu3/online
  wait
  # then the same sleep loop again

Temporary debug dmesg counts per run:
  empty_bucket  - collect_expired_timers() saw pending_map bit set
                  with an empty hlist (next_expiry had been pulled to
                  a stale empty bucket after re-online)
  stale_pending - timers_dead_cpu() saw non-empty pending_map after
                  migrate (same probe on both builds; Build A then
                  clears, Build B does not)

Build A: clear pending_map after migrate
Build B: do not clear

Build A session 1 / 2 (empty_bucket, stale_pending):
  0,6  0,6  0,6  0,6  0,5
  0,7  0,5  0,5  0,5  0,5

Build B session 1 / 2:
  33,10  18,10  10,10  12,10  6,10
  32,10  14,10  17,10  13,10  10,10

stale_pending > 0 on both builds is expected: migrate leaves bits set
while vectors are empty. Build A clears them; Build B leaves them.
The discriminator is empty_bucket: without clearing it is > 0 every
run; with clearing it stays 0.

Thanks,
Liang Hao