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Date: Sun, 11 Feb 2007 20:48:17 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: oleg@...sign.ru, a.p.zijlstra@...llo.nl, mingo@...e.hu,
hch@...radead.org, akpm@...l.org, stern@...land.harvard.edu
Subject: [RFC PATCH] QRCU fastpath optimization
This patch optimizes the "quick" RCU update-side fastpath, so that in the
absence of readers, synchronize_qrcu() does four non-atomic comparisons
and three memory barriers, eliminating the need to acquire the global
lock in this case. Lightly tested. Algorithm has been validated for
the 3-reader-2-updater and 2-reader-3-updater cases -- 3-readers-3-updaters
case still to be done (I expect to get access to a large-memory machine
in the next few weeks -- need >>20GB).
Not for inclusion. Patch is against Oleg's original patch, and likely
needs to be rediffed against Jen's patchstack. I will do this rediffing
later, first want an easy-to-test and easy-to-inpect version.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
srcu.c | 42 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 37 insertions(+), 5 deletions(-)
diff -urpNa -X dontdiff linux-2.6.19-qrcu/kernel/srcu.c linux-2.6.19-qrcu-fp/kernel/srcu.c
--- linux-2.6.19-qrcu/kernel/srcu.c 2007-02-05 16:27:50.000000000 -0800
+++ linux-2.6.19-qrcu-fp/kernel/srcu.c 2007-02-11 18:10:35.000000000 -0800
@@ -287,23 +287,55 @@ void synchronize_qrcu(struct qrcu_struct
{
int idx;
- smp_mb();
+ smp_mb(); /* Force preceding change to happen before fastpath check. */
+
+ /*
+ * Fastpath: If the two counters sum to "1" at a given point in
+ * time, there are no readers. However, it takes two separate
+ * loads to sample both counters, which won't occur simultaneously.
+ * So we might race with a counter switch, so that we might see
+ * ctr[0]==0, then the counter might switch, then we might see
+ * ctr[1]==1 (unbeknownst to us because there is a reader still
+ * there). So we do a read memory barrier and recheck. If the
+ * same race happens again, there must have been a second counter
+ * switch. This second counter switch could not have happened
+ * until all preceding readers finished, so if the condition
+ * is true both times, we may safely proceed.
+ *
+ * This relies critically on the atomic increment and atomic
+ * decrement being seen as executing in order.
+ */
+
+ if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1) {
+ smp_rmb(); /* Keep two checks independent. */
+ if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1)
+ goto out;
+ }
+
mutex_lock(&qp->mutex);
idx = qp->completed & 0x1;
if (atomic_read(qp->ctr + idx) == 1)
- goto out;
+ goto out_unlock;
atomic_inc(qp->ctr + (idx ^ 0x1));
- /* Reduce the likelihood that qrcu_read_lock() will loop */
+
+ /*
+ * Prevent subsequent decrement from being seen before previous
+ * increment -- such an inversion could cause the fastpath
+ * above to falsely conclude that there were no readers. Also,
+ * reduce the likelihood that qrcu_read_lock() will loop.
+ */
+
smp_mb__after_atomic_inc();
qp->completed++;
atomic_dec(qp->ctr + idx);
__wait_event(qp->wq, !atomic_read(qp->ctr + idx));
-out:
+out_unlock:
mutex_unlock(&qp->mutex);
- smp_mb();
+out:
+ smp_mb(); /* force subsequent free after qrcu_read_unlock(). */
}
EXPORT_SYMBOL_GPL(init_qrcu_struct);
-
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