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Subject[RFC PATCH v4 0/2] sched/fair: Choose the CPU where short task is running during wake up
The main purpose of this change is to avoid too many cross CPU
wake up when it is unnecessary. The frequent cross CPU wake up
brings significant damage to some workloads, especially on high
core count systems.

This patch set inhibits the cross CPU wake-up by placing the wakee
on waking CPU or previous CPU, if both the waker and wakee are
short-duration tasks.

The first patch is to introduce the definition of a short-duration
task. The second patch leverages the first patch to choose a local
or previous CPU for wakee.

Changes since v3:
1. Honglei and Josh have concern that the threshold of short
task duration could be too long. Decreased the threshold from
sysctl_sched_min_granularity to (sysctl_sched_min_granularity / 8),
and the '8' comes from get_update_sysctl_factor().
2. Export p->se.dur_avg to /proc/{pid}/sched per Yicong's suggestion.
3. Move the calculation of average duration from put_prev_task_fair()
to dequeue_task_fair(). Because there is an issue in v3 that,
put_prev_task_fair() will not be invoked by pick_next_task_fair()
in fast path, thus the dur_avg could not be updated timely.
4. Fix the comment in PATCH 2/2, that "WRITE_ONCE(CPU1->ttwu_pending, 1);"
on CPU0 is earlier than CPU1 getting "ttwu_list->p0", per Tianchen.
5. Move the scan for CPU with short duration task from select_idle_cpu()
to select_idle_siblings(), because there is no CPU scan involved, per

Changes since v2:

1. Peter suggested comparing the duration of waker and the cost to
scan for an idle CPU: If the cost is higher than the task duration,
do not waste time finding an idle CPU, choose the local or previous
CPU directly. A prototype was created based on this suggestion.
However, according to the test result, this prototype does not inhibit
the cross CPU wakeup and did not bring improvement. Because the cost
to find an idle CPU is small in the problematic scenario. The root
cause of the problem is a race condition between scanning for an idle
CPU and task enqueue(please refer to the commit log in PATCH 2/2).
So v3 does not change the core logic of v2, with some refinement based
on Peter's suggestion.

2. Simplify the logic to record the task duration per Peter and Abel's suggestion.

This change brings overall improvement on some microbenchmarks, both on
Intel and AMD platforms.


Chen Yu (2):
sched/fair: Introduce short duration task check
sched/fair: Choose the CPU where short task is running during wake up

include/linux/sched.h | 3 +++
kernel/sched/core.c | 2 ++
kernel/sched/debug.c | 1 +
kernel/sched/fair.c | 32 ++++++++++++++++++++++++++++++++
kernel/sched/features.h | 1 +
5 files changed, 39 insertions(+)


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