linux-kernel - Re: [PATCH V2] sched: Forward deadline for early tick

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Message-ID: <CAKfTPtD=OB9i1SZHE1bbmP23shVkrDj52K_V-5KByh3hDJJbcg@mail.gmail.com>
Date: Mon, 20 Jan 2025 14:39:52 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Honglei Wang <jameshongleiwang@....com>
Cc: zihan zhou <15645113830zzh@...il.com>, bsegall@...gle.com, dietmar.eggemann@....com, 
	juri.lelli@...hat.com, linux-kernel@...r.kernel.org, mgorman@...e.de, 
	mingo@...hat.com, peterz@...radead.org, rostedt@...dmis.org, 
	vschneid@...hat.com, yaowenchao@...com, yaozhenguo@...com
Subject: Re: [PATCH V2] sched: Forward deadline for early tick

On Mon, 20 Jan 2025 at 12:44, Honglei Wang <jameshongleiwang@....com> wrote:
>
> Hi Zihan,
>
> On 2025/1/20 15:38, zihan zhou wrote:
> > Thanks for your reply!
> >
> >> For sure we should provide recommendation but I'm not sure it's worth
> >> a warning but better a documentation for user who wants to set their
> >> slice
> >
> >> Could you create a patch that only updates sysctl_sched_base_slice
> >> and normalized_sysctl_sched_base_slice to 700000UL with figures and
> >> explanation
> >
> > Thank you for your suggestion! Adding a warning is not worth it. I will
> > submit a new patch soon, which changes the default value of slice
> > and add the code comments about this change. I am not sure if I
> > should change the files in the Documentation directory. If I should,
> > I will move the content in the comments to the Documentation directory.
> >
> >> sysctl_sched_base_slice is updated by update_sysctl() for each cpu
> >> hotplug so you can't really set it directly
> >>
> >> You can then try a 2nd patch which updates it with debugfs but I'm not
> >> convince at all by this benefit because it should update the
> >> normalized_sysctl_sched_base_slice which is really meaningless for
> >> user
> >
> > There is indeed a big problem with this, and I overlooked it. Thank you
> > for your corrections. I actually intended to add a numerical range
> > requirement to slice through sched_base_slice_fops, because I noticed that
> > syscalls has a range requirement (NSEC_PER_MSEC/10, NSEC_PER_MSEC*100)
> > for modifying slice. However, it seems that the existence of this range is
> > not important.
> >
> >>> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> >>> index ff0e5ab4e37c..d0f90d61398f 100644
> >>> --- a/kernel/sched/syscalls.c
> >>> +++ b/kernel/sched/syscalls.c
> >>> @@ -309,6 +309,10 @@ static void __setscheduler_params(struct task_struct *p,
> >>>                         p->se.slice = clamp_t(u64, attr->sched_runtime,
> >>>                                               NSEC_PER_MSEC/10,   /* HZ=1000 * 10 */
> >>>                                               NSEC_PER_MSEC*100); /* HZ=100  / 10 */
> >>> +
> >>> +                       if (p->se.slice % TICK_NSEC == 0)
> >>> +                               pr_warn("It is not recommended to set the slice to an integer multiple of the tick\n");
> >>
> >> You should better add documentation than a warning
> >>
> >> Also, this code has moved in fair.c now
> >
> > I'm a bit confused here. Has this code been moved from syscalls.c to
> > fair.c file? I haven't seen this change on the master kernel.
> > Is there any other code repository?
> >
>
> Just FYI. I think Vincent meant this one 2cf9ac40073d ("sched/fair:
> Encapsulate set custom slice in a __setparam_fair() function"). You can
> check it at the sched/core branch here:
> https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/core&id=2cf9ac40073ddb6a70dd5ef94f1cf45501b696ed

You can find the information in MAINTAINERS file at the scheduler
section. Actually it's

T: git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core

>
> Thanks,
> Honglei
>