[RFC PATCH 3/3] x86,smp: auto tune spinlock backoff delay factor

[Rik van Riel <riel@redhat.com>]

Subject: x86,smp: auto tune spinlock backoff delay factor

Many spinlocks are embedded in data structures; having many CPUs
pounce on the cache line the lock is in will slow down the lock
holder, and can cause system performance to fall off a cliff.

The paper "Non-scalable locks are dangerous" is a good reference:

	http://pdos.csail.mit.edu/papers/linux:lock.pdf

In the Linux kernel, spinlocks are optimized for the case of
there not being contention. After all, if there is contention,
the data structure can be improved to reduce or eliminate
lock contention.

Likewise, the spinlock API should remain simple, and the
common case of the lock not being contended should remain
as fast as ever.

However, since spinlock contention should be fairly uncommon,
we can add functionality into the spinlock slow path that keeps
system performance from falling off a cliff when there is lock
contention.

Proportional delay in ticket locks is delaying the time between
checking the ticket based on a delay factor, and the number of
CPUs ahead of us in the queue for this lock. Checking the lock
less often allows the lock holder to continue running, resulting
in better throughput and preventing performance from dropping
off a cliff.

Proportional spinlock delay with a high delay factor works well
when there is lots contention on a lock. Likewise, a smaller
delay factor works well when a lock is lightly contended.

Making the code auto-tune the delay factor results in a system
that performs well with both light and heavy lock contention.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 arch/x86/kernel/smp.c |   49 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 1c1eb02..8786476 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -113,19 +113,62 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
 static bool smp_no_nmi_ipi = false;
 
 /*
- * Wait on a congested ticket spinlock.
+ * Wait on a congested ticket spinlock. Many spinlocks are embedded in
+ * data structures; having many CPUs pounce on the cache line with the
+ * spinlock simultaneously can slow down the lock holder, and the system
+ * as a whole.
+ *
+ * To prevent total performance collapse in case of bad spinlock contention,
+ * perform proportional backoff. The per-cpu value of delay is automatically
+ * tuned to limit the number of times spinning CPUs poll the lock before
+ * obtaining it. This limits the amount of cross-CPU traffic required to obtain
+ * a spinlock, and keeps system performance from dropping off a cliff.
+ *
+ * There is a tradeoff. If we poll too often, the whole system is slowed
+ * down. If we sleep too long, the lock will go unused for a period of
+ * time. Adjusting "delay" to poll, on average, 2.7 times before the
+ * lock is obtained seems to result in low bus traffic. The combination
+ * of aiming for a non-integer amount of average polls, and scaling the
+ * sleep period proportionally to how many CPUs are ahead of us in the
+ * queue for this ticket lock seems to reduce the amount of time spent
+ * "oversleeping" the release of the lock.
  */
+#define MIN_SPINLOCK_DELAY 1
+#define MAX_SPINLOCK_DELAY 1000
+DEFINE_PER_CPU(int, spinlock_delay) = { MIN_SPINLOCK_DELAY };
 void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 {
+	/*
+	 * Use the raw per-cpu pointer; preemption is disabled in the
+	 * spinlock code. This avoids put_cpu_var once we have the lock.
+	 */
+	int *delay_ptr = &per_cpu(spinlock_delay, smp_processor_id());
+	int delay = *delay_ptr;
+
 	for (;;) {
-		int loops = 50 * (__ticket_t)(inc.tail - inc.head);
+		int loops = delay * (__ticket_t)(inc.tail - inc.head);
 		while (loops--)
 			cpu_relax();
 
 		inc.head = ACCESS_ONCE(lock->tickets.head);
-		if (inc.head == inc.tail)
+		if (inc.head == inc.tail) {
+			/* Decrease the delay, since we may have overslept. */
+			if (delay > MIN_SPINLOCK_DELAY)
+				delay--;
 			break;
+		}
+
+		/*
+		 * The lock is still busy, the delay was not long enough.
+		 * Going through here 2.7 times will, on average, cancel
+		 * out the decrement above. Using a non-integer number
+		 * gets rid of performance artifacts and reduces oversleeping.
+		 */
+		if (delay < MAX_SPINLOCK_DELAY &&
+				(!(inc.head & 3) == 0 || (inc.head & 7) == 1))
+			delay++;
 	}
+	*delay_ptr = delay;
 }
 
 /*