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Date: Tue, 26 May 2015 09:28:46 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Denys Vlasenko <dvlasenk@...hat.com>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <laijs@...fujitsu.com>,
Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rcu: Deinline rcu_read_lock_sched_held() if
DEBUG_LOCK_ALLOC
On Tue, May 26, 2015 at 12:04:46PM -0400, Steven Rostedt wrote:
> On Tue, 26 May 2015 17:48:34 +0200
> Denys Vlasenko <dvlasenk@...hat.com> wrote:
>
> > DEBUG_LOCK_ALLOC=y is not a production setting, but it is
> > not very unusual either. Many developers routinely
> > use kernels built with it enabled.
> >
> > Apart from being selected by hand, it is also auto-selected by
> > PROVE_LOCKING "Lock debugging: prove locking correctness" and
> > LOCK_STAT "Lock usage statistics" config options.
> > LOCK STAT is necessary for "perf lock" to work.
> >
> > I wouldn't spend too much time optimizing it, but this particular
> > function has a very large cost in code size: when it is deinlined,
> > code size decreases by 830,000 bytes:
> >
> > text data bss dec hex filename
> > 85674192 22294776 20627456 128596424 7aa39c8 vmlinux.before
> > 84837612 22294424 20627456 127759492 79d7484 vmlinux
> >
> > (with this config: http://busybox.net/~vda/kernel_config)
> >
> > Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> > CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > CC: Josh Triplett <josh@...htriplett.org>
> > CC: Steven Rostedt <rostedt@...dmis.org>
> > CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > CC: Lai Jiangshan <laijs@...fujitsu.com>
> > CC: Tejun Heo <tj@...nel.org>
> > CC: Oleg Nesterov <oleg@...hat.com>
> > CC: linux-kernel@...r.kernel.org
>
> Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
Thank you both!
I had to apply by hand -- next time, please base on the rcu/dev branch of
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git.
Please also check to make sure that I didn't mess something up by
my hand-applying, please see below.
Thanx, Paul
------------------------------------------------------------------------
commit 523d371ef86584a28def5d23288babffdbcf5f85
Author: Denys Vlasenko <dvlasenk@...hat.com>
Date: Tue May 26 17:48:34 2015 +0200
rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK_ALLOC
DEBUG_LOCK_ALLOC=y is not a production setting, but it is
not very unusual either. Many developers routinely
use kernels built with it enabled.
Apart from being selected by hand, it is also auto-selected by
PROVE_LOCKING "Lock debugging: prove locking correctness" and
LOCK_STAT "Lock usage statistics" config options.
LOCK STAT is necessary for "perf lock" to work.
I wouldn't spend too much time optimizing it, but this particular
function has a very large cost in code size: when it is deinlined,
code size decreases by 830,000 bytes:
text data bss dec hex filename
85674192 22294776 20627456 128596424 7aa39c8 vmlinux.before
84837612 22294424 20627456 127759492 79d7484 vmlinux
(with this config: http://busybox.net/~vda/kernel_config)
Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: Josh Triplett <josh@...htriplett.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: Lai Jiangshan <laijs@...fujitsu.com>
CC: Tejun Heo <tj@...nel.org>
CC: Oleg Nesterov <oleg@...hat.com>
CC: linux-kernel@...r.kernel.org
Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 18e377b92875..3a4d1bf430b1 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -467,46 +467,10 @@ int rcu_read_lock_bh_held(void);
* If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
* RCU-sched read-side critical section. In absence of
* CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
- * critical section unless it can prove otherwise. Note that disabling
- * of preemption (including disabling irqs) counts as an RCU-sched
- * read-side critical section. This is useful for debug checks in functions
- * that required that they be called within an RCU-sched read-side
- * critical section.
- *
- * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
- * and while lockdep is disabled.
- *
- * Note that if the CPU is in the idle loop from an RCU point of
- * view (ie: that we are in the section between rcu_idle_enter() and
- * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
- * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs
- * that are in such a section, considering these as in extended quiescent
- * state, so such a CPU is effectively never in an RCU read-side critical
- * section regardless of what RCU primitives it invokes. This state of
- * affairs is required --- we need to keep an RCU-free window in idle
- * where the CPU may possibly enter into low power mode. This way we can
- * notice an extended quiescent state to other CPUs that started a grace
- * period. Otherwise we would delay any grace period as long as we run in
- * the idle task.
- *
- * Similarly, we avoid claiming an SRCU read lock held if the current
- * CPU is offline.
+ * critical section unless it can prove otherwise.
*/
#ifdef CONFIG_PREEMPT_COUNT
-static inline int rcu_read_lock_sched_held(void)
-{
- int lockdep_opinion = 0;
-
- if (!debug_lockdep_rcu_enabled())
- return 1;
- if (!rcu_is_watching())
- return 0;
- if (!rcu_lockdep_current_cpu_online())
- return 0;
- if (debug_locks)
- lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
- return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
-}
+int rcu_read_lock_sched_held(void);
#else /* #ifdef CONFIG_PREEMPT_COUNT */
static inline int rcu_read_lock_sched_held(void)
{
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index afaecb7a799a..fec5f48b8860 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -62,6 +62,55 @@ MODULE_ALIAS("rcupdate");
module_param(rcu_expedited, int, 0);
+#if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
+/**
+ * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
+ *
+ * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an
+ * RCU-sched read-side critical section. In absence of
+ * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
+ * critical section unless it can prove otherwise. Note that disabling
+ * of preemption (including disabling irqs) counts as an RCU-sched
+ * read-side critical section. This is useful for debug checks in functions
+ * that required that they be called within an RCU-sched read-side
+ * critical section.
+ *
+ * Check debug_lockdep_rcu_enabled() to prevent false positives during boot
+ * and while lockdep is disabled.
+ *
+ * Note that if the CPU is in the idle loop from an RCU point of
+ * view (ie: that we are in the section between rcu_idle_enter() and
+ * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU
+ * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs
+ * that are in such a section, considering these as in extended quiescent
+ * state, so such a CPU is effectively never in an RCU read-side critical
+ * section regardless of what RCU primitives it invokes. This state of
+ * affairs is required --- we need to keep an RCU-free window in idle
+ * where the CPU may possibly enter into low power mode. This way we can
+ * notice an extended quiescent state to other CPUs that started a grace
+ * period. Otherwise we would delay any grace period as long as we run in
+ * the idle task.
+ *
+ * Similarly, we avoid claiming an SRCU read lock held if the current
+ * CPU is offline.
+ */
+int rcu_read_lock_sched_held(void)
+{
+ int lockdep_opinion = 0;
+
+ if (!debug_lockdep_rcu_enabled())
+ return 1;
+ if (!rcu_is_watching())
+ return 0;
+ if (!rcu_lockdep_current_cpu_online())
+ return 0;
+ if (debug_locks)
+ lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
+ return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
+}
+EXPORT_SYMBOL(rcu_read_lock_sched_held);
+#endif
+
#ifndef CONFIG_TINY_RCU
static atomic_t rcu_expedited_nesting =
--
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