| Message ID | 20230719103743.4775-2-chrubis@suse.cz |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | Two fixes for sysctl_sched_rr_timeslice | expand |
Hi, [ Cc Shile Zhang ] > 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; It looks like very old bug, from v4.11-rc1. I guess this should go to all stable and LTS kernels. Fixes: 975e155ed873 ("sched/rt: Show the 'sched_rr_timeslice' SCHED_RR timeslice tuning knob in milliseconds") Reviewed-by: Petr Vorel <pvorel@suse.cz> Kind regards, Petr > 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,
On Wed, Jul 19, 2023 at 12:37:42PM +0200, Cyril Hrubis wrote: > 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> Acked-by: Mel Gorman <mgorman@suse.de>
Tested-by: Petr Vorel <pvorel@suse.cz>
Kind regards,
Petr
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,
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(-)