Re: [PATCH V3 1/2] sched: Reduce the default slice to avoid tasks getting an extra tick

[Qais Yousef]

On 02/24/25 15:15, Vincent Guittot wrote:
> On Mon, 10 Feb 2025 at 10:13, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, Feb 10, 2025 at 01:29:31AM +0000, Qais Yousef wrote:
> >
> > > I brought the topic up of these magic values with Peter and Vincent in LPC as
> > > I think this logic is confusing. I have nothing against your patch, but if the
> > > maintainers agree I am in favour of removing it completely in favour of setting
> > > it to a single value that is the same across all systems.
> >
> > You're talking about the scaling, right?
> >
> > Yeah, it is of limited use. The cap at 8, combined with the fact that
> > its really hard to find a machine with less than 8 CPUs on, makes the
> > whole thing mostly useless.
> >
> > Back when we did this, we still had dual-core laptops. Now phones have
> > 8 or more CPUs on.
> >
> > So I don't think I mind ripping it out.
> 
> Beside the question of ripping it out or not. We still have a number
> of devices with less than 8 cores but they are not targeting phones,
> laptops or servers ...

I'm not sure if this is in favour or against the rip out, or highlighting a new
problem. But in case it is against the rip-out, hopefully my answer in [1]
highlights why the relationship to CPU number is actually weak and not really
helping much - I think it is making implicit assumption about the workloads and
I don't think this holds anymore. Ignore me otherwise :-)

FWIW a raspberry PI can be used as a server, a personal computer, a multimedia
entertainment system, a dumb sensor recorder/relayer or anything else. I think
most systems expect to run a variety of workloads and IMHO the fact the system
is overloaded and we need a reasonable default base_slice to ensure timely
progress of all running tasks has little relation to NR_CPUs nowadays.

[1] https://lore.kernel.org/all/20250210230500.53mybtyvzhdagot5@airbuntu/