Messages in this thread |  | | From | Chen Yu <> | Subject | [RFC PATCH v4 0/2] sched/fair: Choose the CPU where short task is running during wake up | Date | Fri, 16 Dec 2022 14:08:50 +0800 |
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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 Yicong.
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.
v3: https://lore.kernel.org/lkml/cover.1669862147.git.yu.c.chen@intel.com/ v2: https://lore.kernel.org/all/cover.1666531576.git.yu.c.chen@intel.com/ v1: https://lore.kernel.org/lkml/20220915165407.1776363-1-yu.c.chen@intel.com/
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(+)
-- 2.25.1
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