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| author | Cyril Hrubis <chrubis@suse.cz> | 2024-02-22 18:05:38 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2024-03-01 13:06:08 +0100 |
| commit | 41b7572dea9f7196d075b40d5ac8aafdb5f4b0d4 (patch) | |
| tree | 357f2912e61b5ad731f62b1f3db5eb4fd2df07f8 | |
| parent | 4a41f41c928d1c62da9fcc87ad349c201d91a43e (diff) | |
| download | linux-stable-41b7572dea9f7196d075b40d5ac8aafdb5f4b0d4.tar.gz linux-stable-41b7572dea9f7196d075b40d5ac8aafdb5f4b0d4.tar.bz2 linux-stable-41b7572dea9f7196d075b40d5ac8aafdb5f4b0d4.zip | |
sched/rt: Fix sysctl_sched_rr_timeslice intial value
[ Upstream commit c7fcb99877f9f542c918509b2801065adcaf46fa ]
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@suse.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Tested-by: Petr Vorel <pvorel@suse.cz>
Link: https://lore.kernel.org/r/20230802151906.25258-2-chrubis@suse.cz
[ pvorel: rebased for 4.19 ]
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
| -rw-r--r-- | kernel/sched/rt.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 394c66442cff..ce4594215728 100644 --- 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; static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun); |