Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity for managed interrupt

[Marc Zyngier]

On 2019-12-20 23:31, Ming Lei wrote:
> On Fri, Dec 20, 2019 at 03:38:24PM +0000, John Garry wrote:
> > > > We've got some more results and it looks promising.
> > > >
> > > > So with your patch we get a performance boost of 3180.1K -> 
> > 3294.9K
> > > > IOPS in the D06 SAS env. Then when we change the driver to use
> > > > threaded interrupt handler (mainline currently uses tasklet), we 
> > get a
> > > > boost again up to 3415K IOPS.
> > > >
> > > > Now this is essentially the same figure we had with using 
> > threaded
> > > > handler + the gen irq change in spreading the handler CPU 
> > affinity. We
> > > > did also test your patch + gen irq change and got a performance 
> > drop,
> > > > to 3347K IOPS.
> > > >
> > > > So tentatively I'd say your patch may be all we need.
> > >
> > > OK.
> > >
> > > > FYI, here is how the effective affinity is looking for both SAS
> > > > controllers with your patch:
> > > >
> > > > 74:02.0
> > > > irq 81, cpu list 24-29, effective list 24 cq
> > > > irq 82, cpu list 30-35, effective list 30 cq
> > >
> > > Cool.
> > >
> > > [...]
> > >
> > > > As for your patch itself, I'm still concerned of possible 
> > regressions
> > > > if we don't apply this effective interrupt affinity spread 
> > policy to
> > > > only managed interrupts.
> > >
> > > I'll try and revise that as I post the patch, probably at some 
> > point
> > > between now and Christmas. I still think we should find a way to
> > > address this for the D05 SAS driver though, maybe by managing the
> > > affinity yourself in the driver. But this requires 
> > experimentation.
> >
> > I've already done something experimental for the driver to manage 
> > the
> > affinity, and performance is generally much better:
> >
> >
> > https://github.com/hisilicon/kernel-dev/commit/e15bd404ed1086fed44da34ed3bd37a8433688a7
> >
> > But I still think it's wise to only consider managed interrupts for 
> > now.
> >
> > >
> > > > JFYI, about NVMe CPU lockup issue, there are 2 works on going 
> > here:
> > > >
> > > > 
> > https://lore.kernel.org/linux-nvme/20191209175622.1964-1-kbusch@xxxxxxxxxx/T/#t
> > > >
> > > >
> > > > 
> > https://lore.kernel.org/linux-block/20191218071942.22336-1-ming.lei@xxxxxxxxxx/T/#t
> > > >
> > >
> > > I've also managed to trigger some of them now that I have access 
> > to
> > > a decent box with nvme storage.
> >
> > I only have 2x NVMe SSDs when this occurs - I should not be hitting 
> > this...
> >
> > Out of curiosity, have you tried
> > > with the SMMU disabled? I'm wondering whether we hit some livelock
> > > condition on unmapping buffers...
> >
> > No, but I can give it a try. Doing that should lower the CPU usage, 
> > though,
> > so maybe masks the issue - probably not.
>
> Lots of CPU lockup can is performance issue if there isn't obvious 
> bug.
>
> I am wondering if you may explain it a bit why enabling SMMU may save
> CPU a it?

The other way around. mapping/unmapping IOVAs doesn't comes for free.
I'm trying to find out whether the NVMe map/unmap patterns trigger
something unexpected in the SMMU driver, but that's a very long shot.

        M.
--
Jazz is not dead. It just smells funny...