linux-kernel - Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

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Date:	Wed, 18 Jul 2012 15:07:06 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, avi@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

On Wed, Jul 18, 2012 at 02:48:44PM +0300, Gleb Natapov wrote:
> On Wed, Jul 18, 2012 at 02:39:10PM +0300, Michael S. Tsirkin wrote:
> > On Wed, Jul 18, 2012 at 02:22:19PM +0300, Michael S. Tsirkin wrote:
> > > > > > > > > So as was discussed kvm_set_irq under spinlock is bad for scalability
> > > > > > > > > with multiple VCPUs.  Why do we need a spinlock simply to protect
> > > > > > > > > level_asserted?  Let's use an atomic test and set/test and clear and the
> > > > > > > > > problem goes away.
> > > > > > > > > 
> > > > > > > > That sad reality is that for level interrupt we already scan all vcpus
> > > > > > > > under spinlock.
> > > > > > > 
> > > > > > > Where?
> > > > > > > 
> > > > > > ioapic
> > > > > 
> > > > > $ grep kvm_for_each_vcpu virt/kvm/ioapic.c
> > > > > $
> > > > > 
> > > > > ?
> > > > > 
> > > > 
> > > > Come on Michael. You can do better than grep and actually look at what
> > > > code does. The code that loops over all vcpus while delivering an irq is
> > > > in kvm_irq_delivery_to_apic(). Now grep for that.
> > > 
> > > Hmm, I see, it's actually done for edge if injected from ioapic too,
> > > right?
> > > 
> > > So set_irq does a linear scan, and for each matching CPU it calls
> > > kvm_irq_delivery_to_apic which is another scan?
> > > So it's actually N^2 worst case for a broadcast?
> > 
> > No it isn't, I misread the code.
> > 
> > 
> > Anyway, maybe not trivially but this looks fixable to me: we could drop
> > the ioapic lock before calling kvm_irq_delivery_to_apic.
> > 
> May be, may be not. Just saying "lets drop lock whenever we don't feel
> like holding one" does not cut it.

One thing we do is set remote_irr if interrupt was injected.
I agree these things are tricky.

One other question:

static int ioapic_service(struct kvm_ioapic *ioapic, unsigned int idx)
{
        union kvm_ioapic_redirect_entry *pent;
        int injected = -1;

        pent = &ioapic->redirtbl[idx];

        if (!pent->fields.mask) {
                injected = ioapic_deliver(ioapic, idx);
                if (injected && pent->fields.trig_mode == IOAPIC_LEVEL_TRIG)
                        pent->fields.remote_irr = 1;
        }

        return injected;
}


This if (injected) looks a bit strange since ioapic_deliver returns
-1 if no matching destinations. Should be if (injected > 0)?



> Back to original point though current
> situation is that calling kvm_set_irq() under spinlock is not worse for
> scalability than calling it not under one.

Yes. Still the specific use can just use an atomic flag,
lock+bool is not needed, and we won't need to undo it later.

> --
> 			Gleb.
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