Re: [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support forlinux guests running on KVM hypervisor

[Raghavendra K T]

On 07/25/2013 02:45 PM, Gleb Natapov wrote:
> On Thu, Jul 25, 2013 at 02:47:37PM +0530, Raghavendra K T wrote:
> > On 07/24/2013 06:06 PM, Raghavendra K T wrote:
> > > On 07/24/2013 05:36 PM, Gleb Natapov wrote:
> > > > On Wed, Jul 24, 2013 at 05:30:20PM +0530, Raghavendra K T wrote:
> > > > > On 07/24/2013 04:09 PM, Gleb Natapov wrote:
> > > > > > On Wed, Jul 24, 2013 at 03:15:50PM +0530, Raghavendra K T wrote:
> > > > > > > On 07/23/2013 08:37 PM, Gleb Natapov wrote:
> > > > > > > > On Mon, Jul 22, 2013 at 11:50:16AM +0530, Raghavendra K T wrote:
> > > > > > > > > +static void kvm_lock_spinning(struct arch_spinlock *lock,
> > > > > > > > > __ticket_t want)
> > > > > > > [...]
> > > > > > > > > +
> > > > > > > > > +    /*
> > > > > > > > > +     * halt until it's our turn and kicked. Note that we do safe
> > > > > > > > > halt
> > > > > > > > > +     * for irq enabled case to avoid hang when lock info is
> > > > > > > > > overwritten
> > > > > > > > > +     * in irq spinlock slowpath and no spurious interrupt occur
> > > > > > > > > to save us.
> > > > > > > > > +     */
> > > > > > > > > +    if (arch_irqs_disabled_flags(flags))
> > > > > > > > > +        halt();
> > > > > > > > > +    else
> > > > > > > > > +        safe_halt();
> > > > > > > > > +
> > > > > > > > > +out:
> > > > > > > > So here now interrupts can be either disabled or enabled. Previous
> > > > > > > > version disabled interrupts here, so are we sure it is safe to
> > > > > > > > have them
> > > > > > > > enabled at this point? I do not see any problem yet, will keep
> > > > > > > > thinking.
> > > > > > >
> > > > > > > If we enable interrupt here, then
> > > > > > >
> > > > > > >
> > > > > > > > > +    cpumask_clear_cpu(cpu, &waiting_cpus);
> > > > > > >
> > > > > > > and if we start serving lock for an interrupt that came here,
> > > > > > > cpumask clear and w->lock=null may not happen atomically.
> > > > > > > if irq spinlock does not take slow path we would have non null value
> > > > > > > for lock, but with no information in waitingcpu.
> > > > > > >
> > > > > > > I am still thinking what would be problem with that.
> > > > > > Exactly, for kicker waiting_cpus and w->lock updates are
> > > > > > non atomic anyway.
> > > > > >
> > > > > > > > > +    w->lock = NULL;
> > > > > > > > > +    local_irq_restore(flags);
> > > > > > > > > +    spin_time_accum_blocked(start);
> > > > > > > > > +}
> > > > > > > > > +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
> > > > > > > > > +
> > > > > > > > > +/* Kick vcpu waiting on @lock->head to reach value @ticket */
> > > > > > > > > +static void kvm_unlock_kick(struct arch_spinlock *lock,
> > > > > > > > > __ticket_t ticket)
> > > > > > > > > +{
> > > > > > > > > +    int cpu;
> > > > > > > > > +
> > > > > > > > > +    add_stats(RELEASED_SLOW, 1);
> > > > > > > > > +    for_each_cpu(cpu, &waiting_cpus) {
> > > > > > > > > +        const struct kvm_lock_waiting *w =
> > > > > > > > > &per_cpu(lock_waiting, cpu);
> > > > > > > > > +        if (ACCESS_ONCE(w->lock) == lock &&
> > > > > > > > > +            ACCESS_ONCE(w->want) == ticket) {
> > > > > > > > > +            add_stats(RELEASED_SLOW_KICKED, 1);
> > > > > > > > > +            kvm_kick_cpu(cpu);
> > > > > > > > What about using NMI to wake sleepers? I think it was discussed, but
> > > > > > > > forgot why it was dismissed.
> > > > > > >
> > > > > > > I think I have missed that discussion. 'll go back and check. so
> > > > > > > what is the idea here? we can easily wake up the halted vcpus that
> > > > > > > have interrupt disabled?
> > > > > > We can of course. IIRC the objection was that NMI handling path is very
> > > > > > fragile and handling NMI on each wakeup will be more expensive then
> > > > > > waking up a guest without injecting an event, but it is still
> > > > > > interesting
> > > > > > to see the numbers.
> > > > >
> > > > > Haam, now I remember, We had tried request based mechanism. (new
> > > > > request like REQ_UNHALT) and process that. It had worked, but had some
> > > > > complex hacks in vcpu_enter_guest to avoid guest hang in case of
> > > > > request cleared.  So had left it there..
> > > > >
> > > > > https://lkml.org/lkml/2012/4/30/67
> > > > >
> > > > > But I do not remember performance impact though.
> > > > No, this is something different. Wakeup with NMI does not need KVM
> > > > changes at
> > > > all. Instead of kvm_kick_cpu(cpu) in kvm_unlock_kick you send NMI IPI.
> > >
> > > True. It was not NMI.
> > > just to confirm, are you talking about something like this to be tried ?
> > >
> > > apic->send_IPI_mask(cpumask_of(cpu), APIC_DM_NMI);
> >
> > When I started benchmark, I started seeing
> > "Dazed and confused, but trying to continue" from unknown nmi error
> > handling.
> > Did I miss anything (because we did not register any NMI handler)? or
> > is it that spurious NMIs are trouble because we could get spurious NMIs
> > if next waiter already acquired the lock.
> There is a default NMI handler that tries to detect the reason why NMI
> happened (which is no so easy on x86) and prints this message if it
> fails. You need to add logic to detect spinlock slow path there. Check
> bit in waiting_cpus for instance.

aha.. Okay. will check that.

> > (note: I tried sending APIC_DM_REMRD IPI directly, which worked fine
> > but hypercall way of handling still performed well from the results I
> > saw).
> You mean better? This is strange. Have you ran guest with x2apic?

Had the same doubt. So ran the full benchmark for dbench.
So here is what I saw now. 1x was neck to neck (0.9% for hypercall vs 
0.7% for IPI which should boil to no difference considering the noise
factors) but otherwise, by sending IPI I see few percentage gain in 
overcommit cases.




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