From a26979377bf34534ce5ee2712d2a46157ec61498 Mon Sep 17 00:00:00 2001 From: Mukesh Kumar Chaurasiya Date: Wed, 20 Mar 2024 23:08:16 +0530 Subject: [PATCH] sched/doc: Update documentation for base_slice_ns and CONFIG_HZ relation The tunable base_slice_ns is dependent on CONFIG_HZ (i.e. TICK_NSEC) for any significant performance improvement. The reason being the scheduler tick is not frequent enough to force preemption when base_slice expires in case of: base_slice_ns < TICK_NSEC The below data is of stress-ng: Number of CPU: 1 Stressor threads: 4 Time: 30sec On CONFIG_HZ=1000 | base_slice | avg-run (msec) | context-switches | | ---------- | -------------- | ---------------- | | 3ms | 2.914 | 10342 | | 6ms | 4.857 | 6196 | | 9ms | 6.754 | 4482 | | 12ms | 7.872 | 3802 | | 22ms | 11.294 | 2710 | | 32ms | 13.425 | 2284 | On CONFIG_HZ=100 | base_slice | avg-run (msec) | context-switches | | ---------- | -------------- | ---------------- | | 3ms | 9.144 | 3337 | | 6ms | 9.113 | 3301 | | 9ms | 8.991 | 3315 | | 12ms | 12.935 | 2328 | | 22ms | 16.031 | 1915 | | 32ms | 18.608 | 1622 | base_slice: the value of base_slice in ms avg-run (msec): average time of the stressor threads got on cpu before it got preempted context-switches: number of context switches for the stress-ng process Signed-off-by: Mukesh Kumar Chaurasiya Signed-off-by: Ingo Molnar Cc: Randy Dunlap Link: https://lore.kernel.org/r/20240320173815.927637-2-mchauras@linux.ibm.com --- Documentation/scheduler/sched-design-CFS.rst | 3 +++ 1 file changed, 3 insertions(+) diff --git a/Documentation/scheduler/sched-design-CFS.rst b/Documentation/scheduler/sched-design-CFS.rst index 6cffffe26500..e030876fbd68 100644 --- a/Documentation/scheduler/sched-design-CFS.rst +++ b/Documentation/scheduler/sched-design-CFS.rst @@ -100,6 +100,9 @@ which can be used to tune the scheduler from "desktop" (i.e., low latencies) to "server" (i.e., good batching) workloads. It defaults to a setting suitable for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too. +In case CONFIG_HZ results in base_slice_ns < TICK_NSEC, the value of +base_slice_ns will have little to no impact on the workloads. + Due to its design, the CFS scheduler is not prone to any of the "attacks" that exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c, chew.c, ring-test.c, massive_intr.c all work fine and do not impact