6.4-stable review patch. If anyone has any objections, please let me know.
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From: Cyril Hrubis chrubis@suse.cz
[ 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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 00e0e50741153..185d3d749f6b6 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,