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>
diff --git a/init/Kconfig b/init/Kconfig
index 929adf6..3551824 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -488,6 +488,21 @@
Accept the default if unsure.
+config SRCU_SYNCHRONIZE_DELAY
+ int "Microseconds to delay before waiting for readers"
+ range 0 20
+ default 10
+ help
+ This option controls how long SRCU delays before entering its
+ loop waiting on SRCU readers. The purpose of this loop is
+ to avoid the unconditional context-switch penalty that would
+ otherwise be incurred if there was an active SRCU reader,
+ in a manner similar to adaptive locking schemes. This should
+ be set to be a bit longer than the common-case SRCU read-side
+ critical-section overhead.
+
+ Accept the default if unsure.
+
endmenu # "RCU Subsystem"
config IKCONFIG
diff --git a/kernel/srcu.c b/kernel/srcu.c
index c71e075..98d8c1e 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 @@
* 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);
