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

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Message-ID: <7a961950624c414bb9d0c11c914d5c62@www.loen.fr>
Date:   Tue, 24 Dec 2019 11:20:25 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     John Garry <john.garry@...wei.com>, <tglx@...utronix.de>,
        "chenxiang (M)" <chenxiang66@...ilicon.com>,
        <bigeasy@...utronix.de>, <linux-kernel@...r.kernel.org>,
        <hare@...e.com>, <hch@....de>, <axboe@...nel.dk>,
        <bvanassche@....org>, <peterz@...radead.org>, <mingo@...hat.com>,
        Zhang Yi <yi.zhang@...hat.com>
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity  for managed interrupt

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...