Re: [Xen-devel] [PATCH RFC V11 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

[Gleb Natapov <gleb@xxxxxxxxxx>]

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.

--
                        Gleb.

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