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

[Marc Zyngier]

On 2019-12-24 01:59, Ming Lei wrote:
> On Mon, Dec 23, 2019 at 10:47:07AM +0000, Marc Zyngier wrote:
> > On 2019-12-23 10:26, John Garry wrote:
> > > > > > > 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.
> > >
> > > So I tested v5.5-rc3 with and without the SMMU enabled, and 
> > without
> > > the SMMU enabled I don't get the lockup.
> >
> > OK, so my hunch wasn't completely off... At least we have something
> > to look into.
> >
> > [...]
> >
> > > Obviously this is not conclusive, especially with such limited
> > > testing - 5 minute runs each. The CPU load goes up when disabling 
> > the
> > > SMMU, but that could be attributed to extra throughput (1183K ->
> > > 1539K) loading.
> > >
> > > I do notice that since we complete the NVMe request in irq 
> > context,
> > > we also do the DMA unmap, i.e. talk to the SMMU, in the same 
> > context,
> > > which is less than ideal.
> >
> > It depends on how much overhead invalidating the TLB adds to the
> > equation, but we should be able to do some tracing and find out.
> >
> > > I need to finish for the Christmas break today, so can't check 
> > this
> > > much further ATM.
> >
> > No worries. May I suggest creating a new thread in the new year, 
> > maybe
> > involving Robin and Will as well?
>
> Zhang Yi has observed the CPU lockup issue once when running heavy IO 
> on
> single nvme drive, and please CC him if you have new patch to try.

On which architecture? John was indicating that this also happen on 
x86.

> Then looks the DMA unmap cost is too big on aarch64 if SMMU is 
> involved.

So far, we don't have any data suggesting that this is actually the 
case.
Also, other workloads (such as networking) do not exhibit this 
behaviour,
while being least as unmap-heavy as NVMe is.

If the cross-architecture aspect is confirmed, this points more into
the direction of an interaction between the NVMe subsystem and the
DMA API more than an architecture-specific problem.

Given that we have so far very little data, I'd hold off any 
conclusion.

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