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 <mchauras@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20240320173815.927637-2-mchauras@linux.ibm.com

diff --git a/Documentation/scheduler/sched-design-CFS.rst b/Documentation/scheduler/sched-design-CFS.rst
index 6cffffe265006e8b71751649dc06e99852fb507e..e030876fbd689265b276f72a0126c08c2facd1c6 100644 (file)
--- 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