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

[Valentin Schneider <valentin.schneider@arm.com>]

On 21/04/20 13:13, Qais Yousef wrote:
> On 04/14/20 19:58, Valentin Schneider wrote:
>>
>> I'm a bit wary about such blanket changes. I feel like most places impacted
>> by this change don't gain anything by using the random thing. In sched land
>> that would be:
>
> The API has always been clear that cpumask_any return a random cpu within the
> mask. And the fact it's a one liner with cpumask_first() directly visible,
> a user made the choice to stick to cpumask_any() indicates that that's what
> they wanted.
>
> Probably a lot of them they don't care what cpu is returned and happy with the
> random value. I don't see why it has to have an effect. Some could benefit,
> like my use case here. Or others truly don't care, then it's fine to return
> anything, as requested.
>

Exactly, *some* (which AFAICT is a minority) might benefit. So why should
all the others pay the price for a functionality they do not need?

I don't think your change would actually cause a splat somewhere; my point
is about changing existing behaviour without having a story for it. The
thing said 'pick a "random" cpu', sure, but it never did that, it always
picked the first.

I've pointed out two examples that want to be cpumask_first(), and I'm
absolutely certain there are more than these two out there. What if folks
ran some performance test and were completely fine with the _first()
behaviour? What tells you randomness won't degrade some cases?

IMO the correct procedure is to keep everything as it is and improve the
specific callsites that benefit from randomness. I get your point that
using cpumask_any() should be a good enough indicator of the latter, but I
don't think it can realistically be followed. To give my PoV, if in the
past someone had used a cpumask_any() where a cpumask_first() could do, I
would've acked it (disclaimer: super representative population of sample
size = 1).

Flipping the switch on everyone to then have a series of patches "oh this
one didn't need it", "this one neither", "I actually need this to be the
first" just feels sloppy.

> 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.
>

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.