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

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Date:   Mon, 23 Dec 2019 09:07:39 +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>
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity  for managed interrupt

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@kernel.org/T/#t
>> > >
>> > >
>> > > 
>> https://lore.kernel.org/linux-block/20191218071942.22336-1-ming.lei@redhat.com/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...