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

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Message-Id: <20250120073857.47043-1-15645113830zzh@gmail.com>
Date: Mon, 20 Jan 2025 15:38:59 +0800
From: zihan zhou <15645113830zzh@...il.com>
To: vincent.guittot@...aro.org
Cc: 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

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?