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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2010-10-26 13:11:40 +0400
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2010-11-30 09:02:40 +0300
commit46fdb0937f26124700fc9fc80da4776330cc00d3 (patch)
treece3bdf6c0379fdab8c72085f885402751fadea52
parentdb3a8920995484e5e9a0abaf3bad2c7311b163db (diff)
downloadlinux-46fdb0937f26124700fc9fc80da4776330cc00d3.tar.xz
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>
-rw-r--r--init/Kconfig15
-rw-r--r--kernel/srcu.c8
2 files changed, 22 insertions, 1 deletions
diff --git a/init/Kconfig b/init/Kconfig
index 929adf6cb6b4..35518243c4bd 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -488,6 +488,21 @@ config RCU_BOOST_DELAY
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 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);