This is a note to let you know that I've just added the patch titled
sched/rt: Fix sysctl_sched_rr_timeslice intial value
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-rt-fix-sysctl_sched_rr_timeslice-intial-value.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From c7fcb99877f9f542c918509b2801065adcaf46fa Mon Sep 17 00:00:00 2001
From: Cyril Hrubis <chrubis@xxxxxxx>
Date: Wed, 2 Aug 2023 17:19:05 +0200
Subject: sched/rt: Fix sysctl_sched_rr_timeslice intial value
From: Cyril Hrubis <chrubis@xxxxxxx>
commit c7fcb99877f9f542c918509b2801065adcaf46fa upstream.
There is a 10% rounding error in the intial value of the
sysctl_sched_rr_timeslice with CONFIG_HZ_300=y.
This was found with LTP test sched_rr_get_interval01:
sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
sched_rr_get_interval01.c:57: TPASS: sched_rr_get_interval() passed
sched_rr_get_interval01.c:64: TPASS: Time quantum 0s 99999990ns
sched_rr_get_interval01.c:72: TFAIL: /proc/sys/kernel/sched_rr_timeslice_ms != 100 got 90
What this test does is to compare the return value from the
sched_rr_get_interval() and the sched_rr_timeslice_ms sysctl file and
fails if they do not match.
The problem it found is the intial sysctl file value which was computed as:
static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
which works fine as long as MSEC_PER_SEC is multiple of HZ, however it
introduces 10% rounding error for CONFIG_HZ_300:
(MSEC_PER_SEC / HZ) * (100 * HZ / 1000)
(1000 / 300) * (100 * 300 / 1000)
3 * 30 = 90
This can be easily fixed by reversing the order of the multiplication
and division. After this fix we get:
(MSEC_PER_SEC * (100 * HZ / 1000)) / HZ
(1000 * (100 * 300 / 1000)) / 300
(1000 * 30) / 300 = 100
Fixes: 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds")
Signed-off-by: Cyril Hrubis <chrubis@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Petr Vorel <pvorel@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Tested-by: Petr Vorel <pvorel@xxxxxxx>
Link: https://lore.kernel.org/r/20230802151906.25258-2-chrubis@xxxxxxx
[ pvorel: rebased for 5.15, 5.10 ]
Signed-off-by: Petr Vorel <pvorel@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/rt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -8,7 +8,7 @@
#include "pelt.h"
int sched_rr_timeslice = RR_TIMESLICE;
-int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
+int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ;
/* More than 4 hours if BW_SHIFT equals 20. */
static const u64 max_rt_runtime = MAX_BW;
Patches currently in stable-queue which might be from chrubis@xxxxxxx are
queue-5.10/sched-rt-disallow-writing-invalid-values-to-sched_rt_period_us.patch
queue-5.10/sched-rt-fix-sysctl_sched_rr_timeslice-intial-value.patch
queue-5.10/sched-rt-sysctl_sched_rr_timeslice-show-default-timeslice-after-reset.patch
