Re: [Xen-devel] [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

[Gleb Natapov <gleb@xxxxxxxxxx>]

On Tue, Jul 16, 2013 at 09:07:53AM +0530, Raghavendra K T wrote:
> On 07/15/2013 04:06 PM, Gleb Natapov wrote:
> >On Mon, Jul 15, 2013 at 03:20:06PM +0530, Raghavendra K T wrote:
> >>On 07/14/2013 06:42 PM, Gleb Natapov wrote:
> >>>On Mon, Jun 24, 2013 at 06:13:42PM +0530, Raghavendra K T wrote:
> >>>>kvm : Paravirtual ticketlocks support for linux guests running on KVM 
> >>>>hypervisor
> >>>>
> >>>>From: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
> >>>>
> >>trimming
> >>[...]
> >>>>+
> >>>>+static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t 
> >>>>want)
> >>>>+{
> >>>>+ struct kvm_lock_waiting *w;
> >>>>+ int cpu;
> >>>>+ u64 start;
> >>>>+ unsigned long flags;
> >>>>+
> >>>>+ w = &__get_cpu_var(lock_waiting);
> >>>>+ cpu = smp_processor_id();
> >>>>+ start = spin_time_start();
> >>>>+
> >>>>+ /*
> >>>>+  * Make sure an interrupt handler can't upset things in a
> >>>>+  * partially setup state.
> >>>>+  */
> >>>>+ local_irq_save(flags);
> >>>>+
> >>>>+ /*
> >>>>+  * The ordering protocol on this is that the "lock" pointer
> >>>>+  * may only be set non-NULL if the "want" ticket is correct.
> >>>>+  * If we're updating "want", we must first clear "lock".
> >>>>+  */
> >>>>+ w->lock = NULL;
> >>>>+ smp_wmb();
> >>>>+ w->want = want;
> >>>>+ smp_wmb();
> >>>>+ w->lock = lock;
> >>>>+
> >>>>+ add_stats(TAKEN_SLOW, 1);
> >>>>+
> >>>>+ /*
> >>>>+  * This uses set_bit, which is atomic but we should not rely on its
> >>>>+  * reordering gurantees. So barrier is needed after this call.
> >>>>+  */
> >>>>+ cpumask_set_cpu(cpu, &waiting_cpus);
> >>>>+
> >>>>+ barrier();
> >>>>+
> >>>>+ /*
> >>>>+  * Mark entry to slowpath before doing the pickup test to make
> >>>>+  * sure we don't deadlock with an unlocker.
> >>>>+  */
> >>>>+ __ticket_enter_slowpath(lock);
> >>>>+
> >>>>+ /*
> >>>>+  * check again make sure it didn't become free while
> >>>>+  * we weren't looking.
> >>>>+  */
> >>>>+ if (ACCESS_ONCE(lock->tickets.head) == want) {
> >>>>+         add_stats(TAKEN_SLOW_PICKUP, 1);
> >>>>+         goto out;
> >>>>+ }
> >>>>+
> >>>>+ /* Allow interrupts while blocked */
> >>>>+ local_irq_restore(flags);
> >>>>+
> >>>So what happens if an interrupt comes here and an interrupt handler
> >>>takes another spinlock that goes into the slow path? As far as I see
> >>>lock_waiting will become overwritten and cpu will be cleared from
> >>>waiting_cpus bitmap by nested kvm_lock_spinning(), so when halt is
> >>>called here after returning from the interrupt handler nobody is going
> >>>to wake this lock holder. Next random interrupt will "fix" it, but it
> >>>may be several milliseconds away, or never. We should probably check
> >>>if interrupt were enabled and call native_safe_halt() here.
> >>>
> >>
> >>Okay you mean something like below should be done.
> >>if irq_enabled()
> >>   native_safe_halt()
> >>else
> >>   halt()
> >>
> >>It is been a complex stuff for analysis for me.
> >>
> >>So in our discussion stack would looking like this.
> >>
> >>spinlock()
> >>   kvm_lock_spinning()
> >>                   <------ interrupt here
> >>           halt()
> >>
> >>
> >> From the halt if we trace
> >>
> >It is to early to trace the halt since it was not executed yet. Guest
> >stack trace will look something like this:
> >
> >spinlock(a)
> >   kvm_lock_spinning(a)
> >    lock_waiting = a
> >    set bit in waiting_cpus
> >                 <------ interrupt here
> >                 spinlock(b)
> >                   kvm_lock_spinning(b)
> >                     lock_waiting = b
> >                     set bit in waiting_cpus
> >                     halt()
> >                     unset bit in waiting_cpus
> >                     lock_waiting = NULL
> >      ----------> ret from interrupt
> >    halt()
> >
> >Now at the time of the last halt above lock_waiting == NULL and
> >waiting_cpus is empty and not interrupt it pending, so who will unhalt
> >the waiter?
> >
> 
> Yes. if an interrupt occurs between
> local_irq_restore() and halt(), this is possible. and since this is
> rarest of rare (possiility of irq entering slowpath and then no
> random irq to do spurious wakeup), we had never hit this problem in
> the past.
I do not think it is very rare to get interrupt between
local_irq_restore() and halt() under load since any interrupt that
occurs between local_irq_save() and local_irq_restore() will be delivered
immediately after local_irq_restore(). Of course the chance of no other
random interrupt waking lock waiter is very low, but waiter can sleep
for much longer then needed and this will be noticeable in performance.

BTW can NMI handler take spinlocks? If it can what happens if NMI is
delivered in a section protected by local_irq_save()/local_irq_restore()?

> 
> So I am,
> 1. trying to artificially reproduce this.
> 
> 2. I replaced the halt with below code,
>        if (arch_irqs_disabled())
>                 halt();
> 
> and ran benchmarks.
> But this results in degradation because, it means we again go back
> and spin in irq enabled case.
> 
Yes, this is not what I proposed.

> 3. Now I am analyzing the performance overhead of safe_halt in irq
> enabled case.
>       if (arch_irqs_disabled())
>                halt();
>       else
>                safe_halt();
Use of arch_irqs_disabled() is incorrect here. If you are doing it before
local_irq_restore() it will always be false since you disabled interrupt
yourself, if you do it after then it is to late since interrupt can come
between local_irq_restore() and halt() so enabling interrupt and halt
are still not atomic.  You should drop local_irq_restore() and do

  if (arch_irqs_disabled_flags(flags))
        halt();
  else
        safe_halt();

instead.

--
                        Gleb.

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