Re: [PATCH 0/4] sched/rt: Distribute tasks in find_lowest_rq()

[Qais Yousef]

On 04/21/20 15:28, Marc Zyngier wrote:
> On 2020-04-21 15:22, Qais Yousef wrote:
> > On 04/21/20 15:09, Marc Zyngier wrote:
> > > On 2020-04-21 14:18, Valentin Schneider wrote:
> > > > On 21/04/20 13:13, Qais Yousef wrote:
> > > 
> > > [...]
> > > 
> > > > > I CCed Marc who's the maintainer of this file who can clarify better
> > > > > if this
> > > > > really breaks anything.
> > > > >
> > > > > If any interrupt expects to be affined to a specific CPU then this
> > > > > must be
> > > > > described in DT/driver. I think the GIC controller is free to
> > > > > distribute them
> > > > > to any cpu otherwise if !force. Which is usually done by
> > > > > irq_balancer anyway
> > > > > in userspace, IIUC.
> > > > >
> > > > > I don't see how cpumask_any_and() break anything here too. I
> > > > > actually think it
> > > > > improves on things by better distribute the irqs on the system by
> > > > > default.
> > > 
> > > That's a pretty bold statement. Unfortunately, it isn't universally
> > > true.
> > > Some workload will be very happy with interrupts spread all over the
> > > map,
> > > and some others will suffer from it because, well, it interrupts
> > > userspace.
> > > 
> > > > As you say, if someone wants smarter IRQ affinity they can do
> > > > irq_balancer
> > > > and whatnot. The default kernel policy for now has been to shove
> > > > everything
> > > > on the lowest-numbered CPU, and I see no valid reason to change that.
> > > 
> > > Exactly. I would like to keep the kernel policy as simple as
> > > possible for
> > > non-managed interrupts (managed interrupts are another kettle of fish
> > > entirely).
> > > Userpace is in control to place things "intelligently", so let's not
> > > try and
> > > make the kernel smarter than it strictly needs to be.
> > 
> > Fair enough. But why is it asking for cpumask_any() in the first place?
> 
> Implementation detail. Turn it into cpumask_first_and() if you want.

Will do.

Thanks

--
Qais Yousef