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

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Date:   Sat, 21 Dec 2019 07:31:38 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     John Garry <john.garry@...wei.com>
Cc:     Marc Zyngier <maz@...nel.org>, 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 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?

Thanks,
Ming