Messages in this thread | | | Date | Tue, 21 Apr 2020 07:22:49 -0700 | From | Yury Norov <> | Subject | Re: [PATCH 0/4] sched/rt: Distribute tasks in find_lowest_rq() |
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On Tue, Apr 21, 2020 at 03:28:14PM +0200, Vincent Guittot wrote: > On Tue, 21 Apr 2020 at 15:18, Valentin Schneider > <valentin.schneider@arm.com> wrote: > > > > > > 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? > > I tend to agree that any doesn't mean random and using a random cpu > will create strange behavior > > One example is the irq affinity on b.L system. Right now, the irq are > always pinned to the same CPU (the 1st one which is most probably a > Little) but with your change we can imagine that this will change and > might ever change over 2 consecutives boot if for whatever reason (and > this happen) the drivers are not probed in the same order . At the end > you will run some tests with irq on little and other time irq on big. > And more generally speaking and a SMP system can be impacted because > the irq will not be pinned to the same CPU with always the same other > irqs > > > > > 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 > > I agree with this point > > 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.
My 5 cents. I was also surprised when I found cpumask_any() nailed to the first CPU. But for my use I found it beneficial.
Namely, all system IRQs and other events are targeted to the first CPU which is considered as system maintenance unit. Other CPUs are dedicated to user-specific payloads using task isolation. This approach improves latency a lot.
Systems that have many cores operating in idling/powersave mode probably benefit from the state of things as well - they don't wake up sleeping cores and therefore save power and improve IRQ turnaround.
Thanks, Yury
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