On Wed, 29 Mar 2023, Tejun Heo wrote: > Hello, Hugh. How have you been? > > On Wed, Mar 29, 2023 at 12:22:24PM -0700, Hugh Dickins wrote: > > Hi Tejun, > > Butting in here, I'm fascinated. This is certainly not my area, I know > > nothing about rstat, but this is the first time I ever heard someone > > arguing for more disabling of interrupts rather than less. > > > > An interrupt coming in while holding a contended resource can certainly > > add to latencies, that I accept of course. But until now, I thought it > > was agreed best practice to disable irqs only regretfully, when strictly > > necessary. > > > > If that has changed, I for one want to know about it. How should we > > now judge which spinlocks should disable interrupts and which should not? > > Page table locks are currently my main interest - should those be changed? > > For rstat, it's a simple case because the global lock here wraps around > per-cpu locks which have to be irq-safe, so the only difference we get > between making the global irq-unsafe and keeping it so but releasing > inbetween is: > > Global lock held: G > IRQ disabled: I > Percpu lock held: P > > 1. IRQ unsafe > > GGGGGGGGGGGGGGG~~GGGGG > IIII IIII IIII ~~ IIII > PPPP PPPP PPPP ~~ PPPP > > 2. IRQ safe released inbetween cpus > > GGGG GGGG GGGG ~~ GGGG > IIII IIII IIII ~~ IIII > PPPP PPPP PPPP ~~ PPPP > > #2 seems like the obvious thing to do here given how the lock is used and > each P section may take a bit of time. Many thanks for the detailed response. I'll leave it to the rstat folks, to agree or disagree with your analysis there. > > So, in the rstat case, the choice is, at least to me, obvious, but even for > more generic cases where the bulk of actual work isn't done w/ irq disabled, > I don't think the picture is as simple as "use the least protected variant > possible" anymore because the underlying hardware changed. > > For an SMP kernel running on an UP system, "the least protected variant" is > the obvious choice to make because you don't lose anything by holding a > spinlock longer than necessary. However, as you increase the number of CPUs, > there rises a tradeoff between local irq servicing latency and global lock > contention. > > Imagine a, say, 128 cpu system with a few cores servicing relatively high > frequency interrupts. Let's say there's a mildly hot lock. Usually, it shows > up in the system profile but only just. Let's say something happens and the > irq rate on those cores went up for some reason to the point where it > becomes a rather common occurrence when the lock is held on one of those > cpus, irqs are likely to intervene lengthening how long the lock is held, > sometimes, signficantly. Now because the lock is on average held for much > longer, it become a lot hotter as more CPUs would stall on it and depending > on luck or lack thereof these stalls can span many CPUs on the system for > quite a while. This is actually something we saw in production. > > So, in general, there's a trade off between local irq service latency and > inducing global lock contention when using unprotected locks. With more and > more CPUs, the balance keeps shifting. The balance still very much depends > on the specifics of a given lock but yeah I think it's something we need to > be a lot more careful about now. And this looks a very plausible argument to me: I'll let it sink in. But I hadn't heard that the RT folks were clamouring for more irq disabling: perhaps they partition their machines with more care, and are not devotees of high CPU counts. What I hope is that others will chime in one way or the other - it does sound as if a reappraisal of the balances is overdue. Thanks, Hugh (disabling interrupts for as long as he can)
