[PATCH tip/core/rcu 0/2] config-param reduction and blocked-task warning

[PATCH tip/core/rcu 0/2] config-param reduction and blocked-task warning

[Paul E. McKenney]

Hello!

A couple of fixes:

1.	Revert the ill-considered addition of a configuration parameter.
2.	Avoid a pointless blocked-task warning.

After a bit more testing and review, I believe that these should go into
2.6.38, as they both represent regressions.

							Thanx, Paul

[PATCH RFC tip/core/rcu 1/2] rcu: demote SRCU_SYNCHRONIZE_DELAY from kernel-parameter status

[Paul E. McKenney]

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.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 init/Kconfig  |   15 ---------------
 kernel/srcu.c |   15 +++++++++++++--
 2 files changed, 13 insertions(+), 17 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 526ec1c..e11bc79 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -497,21 +497,6 @@ 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 98d8c1e..73ce23f 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);
 
-- 
1.7.3.2

[PATCH RFC tip/core/rcu 2/2] rcu: avoid pointless blocked-task warnings

[Paul E. McKenney]

From: Paul E. McKenney <paul.mckenney@linaro.org>

If the RCU callback-processing kthread has nothing to do, it parks in
a wait_event().  If RCU remains idle for more than two minutes, the
kernel complains about this.  This commit changes from wait_event()
to wait_event_interruptible() to prevent the kernel from complaining
just because RCU is idle.

Reported-by: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Thomas Weber <weber@corscience.de>
---
 kernel/rcutiny.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 0344937..0c343b9 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -189,7 +189,8 @@ static int rcu_kthread(void *arg)
 	unsigned long flags;
 
 	for (;;) {
-		wait_event(rcu_kthread_wq, have_rcu_kthread_work != 0);
+		wait_event_interruptible(rcu_kthread_wq,
+					 have_rcu_kthread_work != 0);
 		morework = rcu_boost();
 		local_irq_save(flags);
 		work = have_rcu_kthread_work;
-- 
1.7.3.2

Re: [PATCH RFC tip/core/rcu 2/2] rcu: avoid pointless blocked-task warnings

[Thomas Gleixner]

On Thu, 13 Jan 2011, Paul E. McKenney wrote:
> Reported-by: Russell King <rmk@arm.linux.org.uk>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

So finally someone managed to clone you ?

Re: [PATCH RFC tip/core/rcu 2/2] rcu: avoid pointless blocked-task warnings

[Paul E. McKenney]

On Thu, Jan 13, 2011 at 08:29:49PM +0100, Thomas Gleixner wrote:
> On Thu, 13 Jan 2011, Paul E. McKenney wrote:
> > Reported-by: Russell King <rmk@arm.linux.org.uk>
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> So finally someone managed to clone you ?

Thankfully, they only managed to multiplex me.  ;-)

							Thanx, Paul