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authorJeremy Fitzhardinge <jeremy@goop.org>2008-07-07 12:07:53 -0700
committerIngo Molnar <mingo@elte.hu>2008-07-16 11:15:53 +0200
commit2d9e1e2f58b5612aa4eab0ab54c84308a29dbd79 (patch)
tree08e91f7f485a8fa8ec2515f8c706870c0d44044b /include/xen/events.h
parent56397f8dadb40055479a8ffff23f21a890098a31 (diff)
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xen: implement Xen-specific spinlocks
The standard ticket spinlocks are very expensive in a virtual environment, because their performance depends on Xen's scheduler giving vcpus time in the order that they're supposed to take the spinlock. This implements a Xen-specific spinlock, which should be much more efficient. The fast-path is essentially the old Linux-x86 locks, using a single lock byte. The locker decrements the byte; if the result is 0, then they have the lock. If the lock is negative, then locker must spin until the lock is positive again. When there's contention, the locker spin for 2^16[*] iterations waiting to get the lock. If it fails to get the lock in that time, it adds itself to the contention count in the lock and blocks on a per-cpu event channel. When unlocking the spinlock, the locker looks to see if there's anyone blocked waiting for the lock by checking for a non-zero waiter count. If there's a waiter, it traverses the per-cpu "lock_spinners" variable, which contains which lock each CPU is waiting on. It picks one CPU waiting on the lock and sends it an event to wake it up. This allows efficient fast-path spinlock operation, while allowing spinning vcpus to give up their processor time while waiting for a contended lock. [*] 2^16 iterations is threshold at which 98% locks have been taken according to Thomas Friebel's Xen Summit talk "Preventing Guests from Spinning Around". Therefore, we'd expect the lock and unlock slow paths will only be entered 2% of the time. Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Christoph Lameter <clameter@linux-foundation.org> Cc: Petr Tesarik <ptesarik@suse.cz> Cc: Virtualization <virtualization@lists.linux-foundation.org> Cc: Xen devel <xen-devel@lists.xensource.com> Cc: Thomas Friebel <thomas.friebel@amd.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'include/xen/events.h')
-rw-r--r--include/xen/events.h7
1 files changed, 7 insertions, 0 deletions
diff --git a/include/xen/events.h b/include/xen/events.h
index 67c4436554a..4680ff3fbc9 100644
--- a/include/xen/events.h
+++ b/include/xen/events.h
@@ -44,4 +44,11 @@ extern void notify_remote_via_irq(int irq);
extern void xen_irq_resume(void);
+/* Clear an irq's pending state, in preparation for polling on it */
+void xen_clear_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. */
+void xen_poll_irq(int irq);
+
#endif /* _XEN_EVENTS_H */