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Message-ID: <20230720095757.syyntxzoqgiucxng@suse.de>
Date: Thu, 20 Jul 2023 10:57:57 +0100
From: Mel Gorman <mgorman@...e.de>
To: Cyril Hrubis <chrubis@...e.cz>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
linux-kernel@...r.kernel.org, ltp@...ts.linux.it,
Jiri Bohac <jbohac@...e.cz>
Subject: Re: [PATCH 1/2] sched/rt: Fix sysctl_sched_rr_timeslice intial value
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@...e.cz>
> CC: Jiri Bohac <jbohac@...e.cz>
Acked-by: Mel Gorman <mgorman@...e.de>
--
Mel Gorman
SUSE Labs
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