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Message-ID: <169202568796.27769.5144994079483945988.tip-bot2@tip-bot2>
Date: Mon, 14 Aug 2023 15:08:07 -0000
From: "tip-bot2 for Cyril Hrubis" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Cyril Hrubis <chrubis@...e.cz>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Petr Vorel <pvorel@...e.cz>, Mel Gorman <mgorman@...e.de>,
x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/rt: Fix sysctl_sched_rr_timeslice intial value
The following commit has been merged into the sched/core branch of tip:
Commit-ID: c7fcb99877f9f542c918509b2801065adcaf46fa
Gitweb: https://git.kernel.org/tip/c7fcb99877f9f542c918509b2801065adcaf46fa
Author: Cyril Hrubis <chrubis@...e.cz>
AuthorDate: Wed, 02 Aug 2023 17:19:05 +02:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 14 Aug 2023 17:01:23 +02:00
sched/rt: Fix sysctl_sched_rr_timeslice intial value
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@...e.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Petr Vorel <pvorel@...e.cz>
Acked-by: Mel Gorman <mgorman@...e.de>
Tested-by: Petr Vorel <pvorel@...e.cz>
Link: https://lore.kernel.org/r/20230802151906.25258-2-chrubis@suse.cz
---
kernel/sched/rt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 00e0e50..185d3d7 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -25,7 +25,7 @@ unsigned int sysctl_sched_rt_period = 1000000;
int sysctl_sched_rt_runtime = 950000;
#ifdef CONFIG_SYSCTL
-static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
+static int sysctl_sched_rr_timeslice = (MSEC_PER_SEC * RR_TIMESLICE) / HZ;
static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
size_t *lenp, loff_t *ppos);
static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,
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