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

From: Marc Zyngier
Date: Tue Apr 21 2020 - 10:09:40 EST


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.

Thanks,

M.
--
Jazz is not dead. It just smells funny...