Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock

[Hugh Dickins <hughd@google.com>]

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)