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Date:   Wed, 25 Dec 2019 08:48:22 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Marc Zyngier <maz@kernel.org>
Cc:     John Garry <john.garry@huawei.com>, tglx@linutronix.de,
        "chenxiang (M)" <chenxiang66@hisilicon.com>, bigeasy@linutronix.de,
        linux-kernel@vger.kernel.org, hare@suse.com, hch@lst.de,
        axboe@kernel.dk, bvanassche@acm.org, peterz@infradead.org,
        mingo@redhat.com, Zhang Yi <yi.zhang@redhat.com>
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity
  for managed interrupt
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On Tue, Dec 24, 2019 at 11:20:25AM +0000, Marc Zyngier wrote:
> 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.

ARM64.

To be honest, I never see such CPU lockup issue on x86 in case of running
heavy IO on single NVMe drive.

> 
> > 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.

Maybe it is because networking workloads usually completes IO in softirq
context, instead of hard interrupt context.

> 
> 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.

We can start to collect latency data of dma unmapping vs nvme_irq()
on both x86 and arm64.

I will see if I can get a such box for collecting the latency data.


Thanks,
Ming