Messages in this thread Patch in this message | | | From | Cyril Hrubis <> | Subject | [PATCH 1/2] sched/rt: Fix sysctl_sched_rr_timeslice intial value | Date | Wed, 19 Jul 2023 12:37:42 +0200 |
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Thre is 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 prolem 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
Signed-off-by: Cyril Hrubis <chrubis@suse.cz> CC: Jiri Bohac <jbohac@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 00e0e5074115..185d3d749f6b 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, -- 2.41.0
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