Re: About threaded interrupt handler CPU affinity

[Thomas Gleixner]

John,

On Mon, 22 Jul 2019, John Garry wrote:
> On 22/07/2019 15:41, Marc Zyngier wrote:
> > On 22/07/2019 15:14, John Garry wrote:
> > > I have a question on commit cbf8699996a6 ("genirq: Let irq thread follow
> > > the effective hard irq affinity"), if you could kindly check:
> > > 
> > > Here we set the thread affinity to be the same as the hard interrupt
> > > affinity. For an arm64 system with GIC ITS, this will be a single CPU,
> > > the lowest in the interrupt affinity mask. So, in this case, effectively
> > > the thread will be bound to a single CPU. I think APIC is the same for
> > > this.
> > > 
> > > The commit message describes the problem that we solve here is that the
> > > thread may become affine to a different CPU to the hard interrupt - does
> > > it mean that the thread CPU mask could not cover that of the hard
> > > interrupt? I couldn't follow the reason.
> > 
> > Assume a 4 CPU system. If the interrupt affinity is on CPU0-1, you could
> > end up with the effective interrupt affinity on CPU0 (which would be
> > typical of the ITS), and the thread running on CPU1. Not great.
> 
> Sure, not great. But the thread can possibly still run on CPU0.

Sure. It could, but it's up to the scheduler to decide. In general it's the
right thing to run the threaded handler on the CPU which handles the
interrupt. With single CPU affinity thats surely a limitation.

> > > We have experimented with fixing the thread mask to be the same as the
> > > interrupt mask (we're using managed interrupts), like before, and get a
> > > significant performance boost at high IO datarates on our storage
> > > controller - like ~11%.
> > 
> > My understanding is that this patch does exactly that. Does it result in
> > a regression?
> 
> Not in the strictest sense for us, I don't know about others. Currently we use
> tasklets, and we find that the CPUs servicing the interrupts (and hence
> tasklets) are heavily loaded. We experience the same for when experimenting
> with threaded interrupt handlers - which would be as expected.
> 
> But, when we make the change as mentioned, our IOPS goes from ~3M -> 3.4M.

So your interrupt is affined to more than one CPU, but due to the ITS
limitation the effective affinity is a single CPU, which in turn restricts
the thread handler affinity to the same single CPU. If you lift that
restriction and let it be affine to the full affinity set of the interrupt
then you get better performance, right? Probably because the other CPU(s)
in the affinity set are less loaded than the one which handles the hard
interrupt.

This is heavily use case dependent I assume, so making this a general
change is perhaps not a good idea, but we could surely make this optional.

Thanks,

	tglx