Re: [PATCH 1/2] sched/rt: Fix sysctl_sched_rr_timeslice intial value

[Mel Gorman <mgorman@suse.de>]

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>

-- 
Mel Gorman
SUSE Labs