linux-kernel - Re: [PATCH] sched/cpufreq: Use USEC_PER

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Message-ID: <ZrtdYXW2VullBiop@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 13 Aug 2024 14:19:29 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Christian Loehle <christian.loehle@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
	Qais Yousef <qyousef@...alina.io>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-pm@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] sched/cpufreq: Use USEC_PER_SEC for deadline task

On 13/08/24 11:17, Christian Loehle wrote:
> On 8/13/24 08:54, Vincent Guittot wrote:
> > On Fri, 9 Aug 2024 at 09:42, Juri Lelli <juri.lelli@...hat.com> wrote:
> >>
> >> On 09/08/24 02:24, Qais Yousef wrote:
> >>> Adding more sched folks to CC
> >>>
> >>> On 08/06/24 14:41, Christian Loehle wrote:
> >>>> Convert the sugov deadline task attributes to use the available
> >>>> definitions to make them more readable.
> >>>> No functional change.
> >>>>
> >>>> Signed-off-by: Christian Loehle <christian.loehle@....com>
> >>>> ---
> >>>>  kernel/sched/cpufreq_schedutil.c | 6 +++---
> >>>>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >>>> index eece6244f9d2..012b38a04894 100644
> >>>> --- a/kernel/sched/cpufreq_schedutil.c
> >>>> +++ b/kernel/sched/cpufreq_schedutil.c
> >>>> @@ -654,9 +654,9 @@ static int sugov_kthread_create(struct sugov_policy *sg_policy)
> >>>>              * Fake (unused) bandwidth; workaround to "fix"
> >>>>              * priority inheritance.
> >>>>              */
> >>>> -           .sched_runtime  =  1000000,
> >>>> -           .sched_deadline = 10000000,
> >>>> -           .sched_period   = 10000000,
> >>>> +           .sched_runtime  = USEC_PER_SEC,
> >>>> +           .sched_deadline = 10 * USEC_PER_SEC,
> >>>> +           .sched_period   = 10 * USEC_PER_SEC,
> >>>
> >>> I think NSEC_PER_MSEC is the correct one. The units in
> >>> include/uapi/linux/sched/types.h is not specified. Had to look at
> >>> sched-deadline.rst to figure it out.
> 
> Huh, that's what I used, see below.
> 
> >>
> >> In practice it's the same number :). But, you are correct, we want
> >> 1ms/10ms and unit is nanoseconds, so NSEC_PER_MSEC.
> > 
> > Yes NSEC_PER_MSEC is the correct unit
> 
> Thank you Qais, Juri and Vincent, but if I'm not missing something we
> have a contradiction.
> This patch should indeed be NSEC_PER_MSEC and I'll send a v2 but:
> - Documentation/scheduler/sched-deadline.rst talks about microseconds:
> SCHED_DEADLINE [18] uses three parameters, named "runtime", "period", and
>  "deadline", to schedule tasks. A SCHED_DEADLINE task should receive
>  "runtime" microseconds of execution time every "period" microseconds, and
>  these "runtime" microseconds are available within "deadline" microseconds
>  from the beginning of the period.
> 
> - sched_setattr / sched_getattr manpages talks about nanoseconds:
>        sched_deadline
>               This field specifies the "Deadline" parameter for deadline
>               scheduling.  The value is expressed in nanoseconds.
> 
> - include/uapi/linux/sched/types.h doesn't mention any unit.
> I will add that to the v2 series.
> 
> - kernel/sched/deadline.c works with nanoseconds internally (although
> with the precision limitation in microseconds).
> 
> No conversion so
> attr->sched_deadline (uapi) == dl_se->dl_deadline (kernel) etc.
> So Documentation/scheduler/sched-deadline.rst is false or what is it that
> I'm missing?
> 

As you say above, internal resolution is essentially down to 1us
(microsecond) and we also check that parameters are at least 1us or
bigger [1].

syscalls and internal mechanics work with nanoseconds, but I don't think
this is a contradiction.

1 - https://elixir.bootlin.com/linux/v6.10.4/source/kernel/sched/deadline.c#L3065