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

[Petr Vorel]

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@xxxxxxx>
> CC: Jiri Bohac <jbohac@xxxxxxx>
> ---
>  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@xxxxxxx>

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,