Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 0/4] sched/rt: Distribute tasks in find_lowest_rq() | Date | Tue, 21 Apr 2020 14:18:10 +0100 |
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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.
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