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Date: Fri, 27 May 2011 23:38:14 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Yinghai Lu <yinghai@...nel.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
tglx@...utronix.de, mingo@...e.hu
Subject: Re: [tip:core/rcu] Revert "rcu: Decrease memory-barrier usage
based on semi-formal proof"
On Fri, May 27, 2011 at 09:03:31PM -0700, Yinghai Lu wrote:
> On 05/27/2011 06:04 PM, Paul E. McKenney wrote:
> >
> > And, after having repeatedly shot myself in the foot trying to make
> > an all-singing all-dancing RCU grace-period latency measurement tool,
> > I fell back to simply measuring the RCU grace-period latency during
> > the time that memory_dev_init() is running. This assumes that the
> > grace periods are started using synchronize_rcu() -- if they are instead
> > being started using call_rcu(), I can adapt to that as well.
> >
> [ 31.635137] cpu_dev_init done
> [ 31.635320] Starting RCU latency diagnostics
> [ 71.605662] Ending RCU latency diagnostics
> [ 71.605924] memory_dev_init done
Thank you! Strange...
Could you please try the following, which replaces the previous
patch?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 3da6a43..f877cf2 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -23,6 +23,7 @@
#include <linux/mutex.h>
#include <linux/stat.h>
#include <linux/slab.h>
+#include <linux/rcupdate.h>
#include <asm/atomic.h>
#include <asm/uaccess.h>
@@ -647,6 +648,7 @@ int __init memory_dev_init(void)
int err;
unsigned long block_sz;
+ trace_rcu_gp_latency_start();
memory_sysdev_class.kset.uevent_ops = &memory_uevent_ops;
ret = sysdev_class_register(&memory_sysdev_class);
if (ret)
@@ -680,5 +682,6 @@ int __init memory_dev_init(void)
out:
if (ret)
printk(KERN_ERR "%s() failed: %d\n", __func__, ret);
+ trace_rcu_gp_latency_stop();
return ret;
}
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index fb2933d..a4abf8b 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -77,6 +77,8 @@ struct rcu_head {
/* Exported common interfaces */
extern void call_rcu_sched(struct rcu_head *head,
void (*func)(struct rcu_head *rcu));
+void trace_rcu_gp_latency_start(void);
+void trace_rcu_gp_latency_stop(void);
extern void synchronize_sched(void);
extern void rcu_barrier_bh(void);
extern void rcu_barrier_sched(void);
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 40d9ed2..58629b5 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -887,6 +887,8 @@ rcu_torture_writer(void *arg)
cur_ops->deferred_free(old_rp);
}
rcutorture_record_progress(++rcu_torture_current_version);
+ if (rcu_torture_current_version == 40)
+ trace_rcu_gp_latency_stop();
oldbatch = cur_ops->completed();
rcu_stutter_wait("rcu_torture_writer");
} while (!kthread_should_stop() && fullstop == FULLSTOP_DONTSTOP);
@@ -1432,6 +1434,7 @@ rcu_torture_init(void)
&sched_ops, &sched_sync_ops, &sched_expedited_ops, };
mutex_lock(&fullstop_mutex);
+ trace_rcu_gp_latency_start();
/* Process args and tell the world that the torturer is on the job. */
for (i = 0; i < ARRAY_SIZE(torture_ops); i++) {
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8b4b3da..49c254b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -889,6 +889,22 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
}
+static int trace_rcu_gp_latency = 0;
+
+void trace_rcu_gp_latency_start(void)
+{
+ printk(KERN_INFO "Starting RCU latency diagnostics\n");
+ trace_rcu_gp_latency = 1;
+}
+EXPORT_SYMBOL_GPL(trace_rcu_gp_latency_start);
+
+void trace_rcu_gp_latency_stop(void)
+{
+ trace_rcu_gp_latency = 0;
+ printk(KERN_INFO "Ending RCU latency diagnostics\n");
+}
+EXPORT_SYMBOL_GPL(trace_rcu_gp_latency_stop);
+
/*
* Report a full set of quiescent states to the specified rcu_state
* data structure. This involves cleaning up after the prior grace
@@ -909,6 +925,9 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
*/
smp_mb(); /* See above block comment. */
gp_duration = jiffies - rsp->gp_start;
+ if (ACCESS_ONCE(trace_rcu_gp_latency))
+ printk(KERN_ALERT
+ "Grace period duration %lu jiffies\n", gp_duration);
if (gp_duration > rsp->gp_max)
rsp->gp_max = gp_duration;
rsp->completed = rsp->gpnum;
@@ -1803,7 +1822,10 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
{
unsigned long flags;
struct rcu_data *rdp;
+ static int i;
+ if (ACCESS_ONCE(trace_rcu_gp_latency) && i++ < 10)
+ dump_stack();
debug_rcu_head_queue(head);
head->func = func;
head->next = NULL;
@@ -1908,10 +1930,13 @@ EXPORT_SYMBOL_GPL(call_rcu_bh);
void synchronize_sched(void)
{
struct rcu_synchronize rcu;
+ ktime_t start, finish;
+ static int i;
if (rcu_blocking_is_gp())
return;
+ start = ktime_get();
init_rcu_head_on_stack(&rcu.head);
init_completion(&rcu.completion);
/* Will wake me after RCU finished. */
@@ -1919,6 +1944,14 @@ void synchronize_sched(void)
/* Wait for it. */
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
+ finish = ktime_get();
+ if (ACCESS_ONCE(trace_rcu_gp_latency)) {
+ printk(KERN_ALERT
+ "synchronize_sched() duration %d microseconds\n",
+ (int)ktime_us_delta(finish, start));
+ if (i++ < 10)
+ dump_stack();
+ }
}
EXPORT_SYMBOL_GPL(synchronize_sched);
--
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