Re: [PATCH 4.19 v2 1/3] sched/rt: Fix sysctl_sched_rr_timeslice intial value

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Feb 22, 2024 at 06:05:38PM +0100, Petr Vorel wrote:
> 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
> [ pvorel: rebased for 4.19 ]
> Signed-off-by: Petr Vorel <pvorel@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 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);
>  
> -- 
> 2.35.3
> 
> 

All now queued up, thanks!

greg k-h