rcu: Make synchronize_srcu_expedited() fast if running readers

The synchronize_srcu_expedited() function is currently quick if there
are no active readers, but will delay a full jiffy if there are any.
If these readers leave their SRCU read-side critical sections quickly,
this is way too long to wait.  So this commit first waits ten microseconds,
and only then falls back to jiffy-at-a-time waiting.

Reported-by: Avi Kivity <avi@redhat.com>
Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Tested-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/srcu.c b/kernel/srcu.c
index c71e07500536..98d8c1e80edb 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -31,6 +31,7 @@
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
+#include <linux/delay.h>
 #include <linux/srcu.h>
 
 static int init_srcu_struct_fields(struct srcu_struct *sp)
@@ -203,9 +204,14 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
          * all srcu_read_lock() calls using the old counters have completed.
          * Their corresponding critical sections might well be still
          * executing, but the srcu_read_lock() primitives themselves
-         * will have finished executing.
+         * 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.
          */
 
+        if (srcu_readers_active_idx(sp, idx))
+                udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY);
         while (srcu_readers_active_idx(sp, idx))
                 schedule_timeout_interruptible(1);