rcu: demote SRCU_SYNCHRONIZE_DELAY from kernel-parameter status

Because the adaptive synchronize_srcu_expedited() approach has
worked very well in testing, remove the kernel parameter and
replace it by a C-preprocessor macro.  If someone finds problems
with this approach, a more complex and aggressively adaptive
approach might be required.

Longer term, SRCU will be merged with the other RCU implementations,
at which point synchronize_srcu_expedited() will be event driven,
just as synchronize_sched_expedited() currently is.  At that point,
there will be no need for this adaptive approach.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/kernel/srcu.c b/kernel/srcu.c
index 98d8c1e80edb..73ce23feaea9 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -156,6 +156,16 @@ void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 
 /*
+ * We use an adaptive strategy for synchronize_srcu() and especially for
+ * synchronize_srcu_expedited().  We spin for a fixed time period
+ * (defined below) to allow SRCU readers to exit their read-side critical
+ * sections.  If there are still some readers after 10 microseconds,
+ * we repeatedly block for 1-millisecond time periods.  This approach
+ * has done well in testing, so there is no need for a config parameter.
+ */
+#define SYNCHRONIZE_SRCU_READER_DELAY 10
+
+/*
  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
  */
 static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
@@ -207,11 +217,12 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
 	 * will have finished executing.  We initially give readers
 	 * an arbitrarily chosen 10 microseconds to get out of their
 	 * SRCU read-side critical sections, then loop waiting 1/HZ
-	 * seconds per iteration.
+	 * seconds per iteration.  The 10-microsecond value has done
+	 * very well in testing.
 	 */
 
 	if (srcu_readers_active_idx(sp, idx))
-		udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY);
+		udelay(SYNCHRONIZE_SRCU_READER_DELAY);
 	while (srcu_readers_active_idx(sp, idx))
 		schedule_timeout_interruptible(1);