linux-kernel - Re: [PATCH v3 0/2] kvm: level irqfd and new eoifd

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Date:	Mon, 16 Jul 2012 08:08:03 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	"mst@...hat.com" <mst@...hat.com>,
	"gleb@...hat.com" <gleb@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 0/2] kvm: level irqfd and new eoifd

On Sun, 2012-07-15 at 13:09 +0300, Avi Kivity wrote:
> On 07/12/2012 08:38 PM, Alex Williamson wrote:
> > On Thu, 2012-07-12 at 10:19 -0600, Alex Williamson wrote:
> >> On Thu, 2012-07-12 at 12:35 +0300, Avi Kivity wrote:
> >> > On 07/11/2012 10:57 PM, Alex Williamson wrote:
> >> > >> 
> >> > >> > We still have classic KVM device assignment to provide fast-path INTx.
> >> > >> > But if we want to replace it midterm, I think it's necessary for VFIO to
> >> > >> > be able to provide such a path as well.
> >> > >> 
> >> > >> I would like VFIO to have no regressions vs. kvm device assignment,
> >> > >> except perhaps in uncommon corner cases.  So I agree.
> >> > > 
> >> > > I ran a few TCP_RR netperf tests forcing a 1Gb tg3 nic to use INTx.
> >> > > Without irqchip support vfio gets a bit more than 60% of KVM device
> >> > > assignment.  That's a little bit of an unfair comparison since it's more
> >> > > than just the I/O path.  With the proposed interfaces here, enabling
> >> > > irqchip, vfio is within 10% of KVM device assignment for INTx.  For MSI,
> >> > > I can actually make vfio come out more than 30% better than KVM device
> >> > > assignment if I send the eventfd from the hard irq handler.  Using a
> >> > > threaded handler as the code currently does, vfio is still behind KVM.
> >> > > It's hard to beat a direct call chain.
> >> > 
> >> > We can have a direct call chain with vfio too, using a custom eventfd
> >> > poll function, no?  Assuming we set up a fast path for unicast msi.
> >> 
> >> You'll have to help me out a little, eventfd_signal walks the wait_queue
> >> and calls each function.  On the injection path that includes
> >> irqfd_wakeup.  For an MSI that seems to already provide direct
> >> injection.  For level we'll schedule_work, so that explains the overhead
> >> in that path, but it's not too dissimilar to a a threaded irq.  vfio
> >> does something very similar, so there's a schedule_work both on inject
> >> and on eoi.  I'll have to check whether anything prevents the unmask
> >> from the wait_queue function in vfio, that could be a significant chunk
> >> of the gap.
> > 
> > Yep, the schedule_work in the eoi is the culprit.  A direct unmask from
> > the wait queue function gives me better results than kvm for INTx.
> > We'll have to see how the leapfrogging goes once KVM switches to
> > injection from the hard handler.  I'm still curious what this custom
> > poll function would give us though.  Thanks,
> > 
> 
> btw, why is the overhead so large?  A context switch should be on the
> order of 1 microsecond or less.  Given that, every 5000 context switches
> per second cost a 1% cpu load on one core.  You would need a very heavy
> interrupt load to see  a large degradation.  Or is the extra latency the
> problem?

I'm using TCP_RR, so latency is the factor.  As I mentioned though, I
have way too much kernel debugging enabled to take these as anything
more than rough estimates.  Thanks,

Alex

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