[Xen-devel] Re: Xen spinlock questions

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Jan Beulich wrote:
> Storing the previous value locally is fine. But I don't think you can do with
> just one 'currently spinning' pointer because of the kicking side
> requirements - if an irq-save lock interrupted an non-irq one (with the
> spinning pointer already set) and a remote CPU releases the lock and
> wants to kick you, it won't be able to if the irq-save lock already replaced
> the non-irq one. Nevertheless, if you're past the try-lock, you may end
> up never getting the wakeup.
>
> Since there can only be one non-irq and one irq-save lock a CPU is
> currently spinning on (the latter as long as you don't re-enable interrupts),
> two fields, otoh, are sufficient.
>   
No, I think it's mostly OK.  The sequence is:

  1. mark that we're about to block
  2. clear pending state on irq
  3. test again with trylock
  4. block in poll

The worrisome interval is between 3-4, but it's only a problem if we end 
up entering the poll with the lock free and the interrupt non-pending.  
There are a few possibilities for what happens in that interval:
  1. the nested spinlock is fastpath only, and takes some other lock
     -> OK, because we're still marked as interested in this lock, and
     will be kicked
     -> if the nested spinlock takes the same lock as the outer one, it
     will end up doing a self-kick
  2. the nested spinlock is slowpath and is kicked
     -> OK, because it will leave the irq pending
  3. the nested spinlock is slowpath, but picks up the lock on retry
     -> bad, because it won't leave the irq pending.

The fix is to make sure that in case 4, it checks to see if it's 
interrupting a blocking lock (by checking if prev != NULL), and leave 
the irq pending if it is.
Updated patch below.  Compile tested only.

> Btw., I also think that using an xchg() (and hence a locked transaction)
> for updating the pointer isn't necessary.
>   
I was concerned about what would happen if an interrupt got between the 
fetch and store.  But thinking about it, I think you're right.
> > > On an 8-core system I'm seeing between 20,000 (x86-64) and 35,000
> > > (i686) wakeup interrupts per CPU. I'm not certain this still counts as rare.
> > > Though that number may go down a little once the hypervisor doesn't
> > > needlessly wake all polling vCPU-s anymore.
> > >   
> > >       
> > What workload are you seeing that on?  20-35k interrupts over what time 
> > period?
> >     
> Oh, sorry, I meant to say that's for a kernel build (-j12), taking about
> 400 wall seconds.
>
>   
> > In my tests, I only see it fall into the slow path a couple of thousand 
> > times per cpu for a kernbench run.
> >     
> Hmm, that's different then for me. Actually, I see a significant spike at
> modpost stage 2, when all the .ko-s get linked. The (spinlock) interrupt
> rate gets up to between 1,000 and 2,000 per CPU and second.
>   
defconfig? allmodconfig?

I'll measure again to confirm.

   J

diff -r 5b4b80c08799 arch/x86/xen/spinlock.c
--- a/arch/x86/xen/spinlock.c   Wed Aug 06 01:35:09 2008 -0700
+++ b/arch/x86/xen/spinlock.c   Wed Aug 06 10:51:27 2008 -0700
@@ -22,6 +22,7 @@
        u64 taken_slow;
        u64 taken_slow_pickup;
        u64 taken_slow_irqenable;
+       u64 taken_slow_spurious;

        u64 released;
        u64 released_slow;
@@ -135,25 +136,41 @@
static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);

-static inline void spinning_lock(struct xen_spinlock *xl)
+/*
+ * Mark a cpu as interested in a lock.  Returns the CPU's previous
+ * lock of interest, in case we got preempted by an interrupt.
+ */
+static inline struct xen_spinlock *spinning_lock(struct xen_spinlock *xl)
{
+       struct xen_spinlock *prev;
+
+       prev = __get_cpu_var(lock_spinners);
        __get_cpu_var(lock_spinners) = xl;
+
        wmb();                  /* set lock of interest before count */
+
        asm(LOCK_PREFIX " incw %0"
            : "+m" (xl->spinners) : : "memory");
+
+       return prev;
}

-static inline void unspinning_lock(struct xen_spinlock *xl)
+/*
+ * Mark a cpu as no longer interested in a lock.  Restores previous
+ * lock of interest (NULL for none).
+ */
+static inline void unspinning_lock(struct xen_spinlock *xl, struct 
xen_spinlock *prev)
{
        asm(LOCK_PREFIX " decw %0"
            : "+m" (xl->spinners) : : "memory");
-       wmb();                  /* decrement count before clearing lock */
-       __get_cpu_var(lock_spinners) = NULL;
+       wmb();                  /* decrement count before restoring lock */
+       __get_cpu_var(lock_spinners) = prev;
}

static noinline int xen_spin_lock_slow(struct raw_spinlock *lock, bool 
irq_enable)
{
        struct xen_spinlock *xl = (struct xen_spinlock *)lock;
+       struct xen_spinlock *prev;
        int irq = __get_cpu_var(lock_kicker_irq);
        int ret;
        unsigned long flags;
@@ -163,33 +180,56 @@
                return 0;

        /* announce we're spinning */
-       spinning_lock(xl);
+       prev = spinning_lock(xl);
+
        flags = __raw_local_save_flags();
        if (irq_enable) {
                ADD_STATS(taken_slow_irqenable, 1);
                raw_local_irq_enable();
        }

-       /* clear pending */
-       xen_clear_irq_pending(irq);
+       ADD_STATS(taken_slow, 1);
+               
+       do {
+               /* clear pending */
+               xen_clear_irq_pending(irq);

-       ADD_STATS(taken_slow, 1);
+               /* check again make sure it didn't become free while
+                  we weren't looking  */
+               ret = xen_spin_trylock(lock);
+               if (ret) {
+                       ADD_STATS(taken_slow_pickup, 1);
+                       
+                       /*
+                        * If we interrupted another spinlock while it
+                        * was blocking, make sure it doesn't block
+                        * without rechecking the lock.
+                        */
+                       if (prev != NULL)
+                               xen_set_irq_pending(irq);
+                       goto out;
+               }

-       /* check again make sure it didn't become free while
-          we weren't looking  */
-       ret = xen_spin_trylock(lock);
-       if (ret) {
-               ADD_STATS(taken_slow_pickup, 1);
-               goto out;
-       }
+		/* 
+		 * Block until irq becomes pending.  If we're
+                * interrupted at this point (after the trylock but
+                * before entering the block), then the nested lock
+                * handler guarantees that the irq will be left
+                * pending if there's any chance the lock became free;
+                * xen_poll_irq() returns immediately if the irq is
+                * pending.
+                */
+               xen_poll_irq(irq);
+               kstat_this_cpu.irqs[irq]++;
+               ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
+       } while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */

-       /* block until irq becomes pending */
-       xen_poll_irq(irq);
-       kstat_this_cpu.irqs[irq]++;
+       /* Leave the irq pending so that any interrupted blocker will
+          recheck. */

out:
        raw_local_irq_restore(flags);
-       unspinning_lock(xl);
+       unspinning_lock(xl, prev);
        return ret;
}

@@ -333,6 +373,8 @@
                           &spinlock_stats.taken_slow_pickup);
        debugfs_create_u64("taken_slow_irqenable", 0444, d_spin_debug,
                           &spinlock_stats.taken_slow_irqenable);
+       debugfs_create_u64("taken_slow_spurious", 0444, d_spin_debug,
+                          &spinlock_stats.taken_slow_spurious);

        debugfs_create_u64("released", 0444, d_spin_debug, 
&spinlock_stats.released);
        debugfs_create_u64("released_slow", 0444, d_spin_debug,
diff -r 5b4b80c08799 drivers/xen/events.c
--- a/drivers/xen/events.c      Wed Aug 06 01:35:09 2008 -0700
+++ b/drivers/xen/events.c      Wed Aug 06 10:51:27 2008 -0700
@@ -162,6 +162,12 @@
{
        struct shared_info *s = HYPERVISOR_shared_info;
        sync_set_bit(port, &s->evtchn_pending[0]);
+}
+
+static inline int test_evtchn(int port)
+{
+       struct shared_info *s = HYPERVISOR_shared_info;
+       return sync_test_bit(port, &s->evtchn_pending[0]);
}


@@ -748,6 +754,25 @@
                clear_evtchn(evtchn);
}

+void xen_set_irq_pending(int irq)
+{
+       int evtchn = evtchn_from_irq(irq);
+
+       if (VALID_EVTCHN(evtchn))
+               set_evtchn(evtchn);
+}
+
+bool xen_test_irq_pending(int irq)
+{
+       int evtchn = evtchn_from_irq(irq);
+       bool ret = false;
+
+       if (VALID_EVTCHN(evtchn))
+               ret = test_evtchn(evtchn);
+
+       return ret;
+}
+
/* Poll waiting for an irq to become pending.  In the usual case, the
   irq will be disabled so it won't deliver an interrupt. */
void xen_poll_irq(int irq)
diff -r 5b4b80c08799 include/xen/events.h
--- a/include/xen/events.h      Wed Aug 06 01:35:09 2008 -0700
+++ b/include/xen/events.h      Wed Aug 06 10:51:27 2008 -0700
@@ -47,6 +47,8 @@

/* Clear an irq's pending state, in preparation for polling on it */
void xen_clear_irq_pending(int irq);
+void xen_set_irq_pending(int irq);
+bool xen_test_irq_pending(int irq);

/* Poll waiting for an irq to become pending.  In the usual case, the
   irq will be disabled so it won't deliver an interrupt. */



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