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Subject[GIT PULL] Re: REGRESSION: Performance regressions from switching anon_vma->lock to mutex

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Jun 15, 2011 at 3:58 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > The first thing that stood out when running it was:
> >
> > 31694 root      20   0 26660 1460 1212 S 17.5  0.0   0:01.97 exim
> >    7 root      -2  19     0    0    0 S 12.7  0.0   0:06.14 rcuc0
> ...
> >
> > Which is an impressive amount of RCU usage..
>
> Gaah. Can we just revert that crazy "use threads for RCU" thing already?

I have this fix queued up currently:

09223371deac: rcu: Use softirq to address performance regression

and that's ready for pulling and should fix this regression:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git core-urgent-for-linus

The revert itself looks quite hairy, i just attempted it: it affects
half a dozen other followup commits. We might be better off using the
(tested) commit above and then shutting down all kthread based
processing and always use a softirq ... or something like that.

if you think that's risky and we should do the revert then i'll
rebase the core/urgent branch and we'll do the revert.

Paul, Peter, what do you think?

Thanks,

Ingo

------------------>
Paul E. McKenney (1):
rcu: Simplify curing of load woes

Shaohua Li (1):
rcu: Use softirq to address performance regression


Documentation/filesystems/proc.txt | 1 +
include/linux/interrupt.h | 1 +
include/trace/events/irq.h | 3 +-
kernel/rcutree.c | 88 ++++++++++++++++-------------------
kernel/rcutree.h | 1 +
kernel/rcutree_plugin.h | 20 ++++----
kernel/softirq.c | 2 +-
tools/perf/util/trace-event-parse.c | 1 +
8 files changed, 57 insertions(+), 60 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index f481780..db3b1ab 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -843,6 +843,7 @@ Provides counts of softirq handlers serviced since boot time, for each cpu.
TASKLET: 0 0 0 290
SCHED: 27035 26983 26971 26746
HRTIMER: 0 0 0 0
+ RCU: 1678 1769 2178 2250


1.3 IDE devices in /proc/ide
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 6c12989..f6efed0 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -414,6 +414,7 @@ enum
TASKLET_SOFTIRQ,
SCHED_SOFTIRQ,
HRTIMER_SOFTIRQ,
+ RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */

NR_SOFTIRQS
};
diff --git a/include/trace/events/irq.h b/include/trace/events/irq.h
index ae045ca..1c09820 100644
--- a/include/trace/events/irq.h
+++ b/include/trace/events/irq.h
@@ -20,7 +20,8 @@ struct softirq_action;
softirq_name(BLOCK_IOPOLL), \
softirq_name(TASKLET), \
softirq_name(SCHED), \
- softirq_name(HRTIMER))
+ softirq_name(HRTIMER), \
+ softirq_name(RCU))

/**
* irq_handler_entry - called immediately before the irq action handler
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 89419ff..ae5c9ea 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -100,6 +100,7 @@ static char rcu_kthreads_spawnable;

static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);
static void invoke_rcu_cpu_kthread(void);
+static void __invoke_rcu_cpu_kthread(void);

#define RCU_KTHREAD_PRIO 1 /* RT priority for per-CPU kthreads. */

@@ -1442,13 +1443,21 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
}

/* If there are callbacks ready, invoke them. */
- rcu_do_batch(rsp, rdp);
+ if (cpu_has_callbacks_ready_to_invoke(rdp))
+ __invoke_rcu_cpu_kthread();
+}
+
+static void rcu_kthread_do_work(void)
+{
+ rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
+ rcu_do_batch(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
+ rcu_preempt_do_callbacks();
}

/*
* Do softirq processing for the current CPU.
*/
-static void rcu_process_callbacks(void)
+static void rcu_process_callbacks(struct softirq_action *unused)
{
__rcu_process_callbacks(&rcu_sched_state,
&__get_cpu_var(rcu_sched_data));
@@ -1465,7 +1474,7 @@ static void rcu_process_callbacks(void)
* the current CPU with interrupts disabled, the rcu_cpu_kthread_task
* cannot disappear out from under us.
*/
-static void invoke_rcu_cpu_kthread(void)
+static void __invoke_rcu_cpu_kthread(void)
{
unsigned long flags;

@@ -1479,6 +1488,11 @@ static void invoke_rcu_cpu_kthread(void)
local_irq_restore(flags);
}

+static void invoke_rcu_cpu_kthread(void)
+{
+ raise_softirq(RCU_SOFTIRQ);
+}
+
/*
* Wake up the specified per-rcu_node-structure kthread.
* Because the per-rcu_node kthreads are immortal, we don't need
@@ -1613,7 +1627,7 @@ static int rcu_cpu_kthread(void *arg)
*workp = 0;
local_irq_restore(flags);
if (work)
- rcu_process_callbacks();
+ rcu_kthread_do_work();
local_bh_enable();
if (*workp != 0)
spincnt++;
@@ -1635,6 +1649,20 @@ static int rcu_cpu_kthread(void *arg)
* to manipulate rcu_cpu_kthread_task. There might be another CPU
* attempting to access it during boot, but the locking in kthread_bind()
* will enforce sufficient ordering.
+ *
+ * Please note that we cannot simply refuse to wake up the per-CPU
+ * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
+ * which can result in softlockup complaints if the task ends up being
+ * idle for more than a couple of minutes.
+ *
+ * However, please note also that we cannot bind the per-CPU kthread to its
+ * CPU until that CPU is fully online. We also cannot wait until the
+ * CPU is fully online before we create its per-CPU kthread, as this would
+ * deadlock the system when CPU notifiers tried waiting for grace
+ * periods. So we bind the per-CPU kthread to its CPU only if the CPU
+ * is online. If its CPU is not yet fully online, then the code in
+ * rcu_cpu_kthread() will wait until it is fully online, and then do
+ * the binding.
*/
static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
{
@@ -1647,12 +1675,14 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", cpu);
if (IS_ERR(t))
return PTR_ERR(t);
- kthread_bind(t, cpu);
+ if (cpu_online(cpu))
+ kthread_bind(t, cpu);
per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
- per_cpu(rcu_cpu_kthread_task, cpu) = t;
sp.sched_priority = RCU_KTHREAD_PRIO;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+ per_cpu(rcu_cpu_kthread_task, cpu) = t;
+ wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
return 0;
}

@@ -1759,12 +1789,11 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
raw_spin_unlock_irqrestore(&rnp->lock, flags);
sp.sched_priority = 99;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+ wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
}
return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
}

-static void rcu_wake_one_boost_kthread(struct rcu_node *rnp);
-
/*
* Spawn all kthreads -- called as soon as the scheduler is running.
*/
@@ -1772,30 +1801,18 @@ static int __init rcu_spawn_kthreads(void)
{
int cpu;
struct rcu_node *rnp;
- struct task_struct *t;

rcu_kthreads_spawnable = 1;
for_each_possible_cpu(cpu) {
per_cpu(rcu_cpu_has_work, cpu) = 0;
- if (cpu_online(cpu)) {
+ if (cpu_online(cpu))
(void)rcu_spawn_one_cpu_kthread(cpu);
- t = per_cpu(rcu_cpu_kthread_task, cpu);
- if (t)
- wake_up_process(t);
- }
}
rnp = rcu_get_root(rcu_state);
(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
- if (rnp->node_kthread_task)
- wake_up_process(rnp->node_kthread_task);
if (NUM_RCU_NODES > 1) {
- rcu_for_each_leaf_node(rcu_state, rnp) {
+ rcu_for_each_leaf_node(rcu_state, rnp)
(void)rcu_spawn_one_node_kthread(rcu_state, rnp);
- t = rnp->node_kthread_task;
- if (t)
- wake_up_process(t);
- rcu_wake_one_boost_kthread(rnp);
- }
}
return 0;
}
@@ -2221,31 +2238,6 @@ static void __cpuinit rcu_prepare_kthreads(int cpu)
}

/*
- * kthread_create() creates threads in TASK_UNINTERRUPTIBLE state,
- * but the RCU threads are woken on demand, and if demand is low this
- * could be a while triggering the hung task watchdog.
- *
- * In order to avoid this, poke all tasks once the CPU is fully
- * up and running.
- */
-static void __cpuinit rcu_online_kthreads(int cpu)
-{
- struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
- struct rcu_node *rnp = rdp->mynode;
- struct task_struct *t;
-
- t = per_cpu(rcu_cpu_kthread_task, cpu);
- if (t)
- wake_up_process(t);
-
- t = rnp->node_kthread_task;
- if (t)
- wake_up_process(t);
-
- rcu_wake_one_boost_kthread(rnp);
-}
-
-/*
* Handle CPU online/offline notification events.
*/
static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
@@ -2262,7 +2254,6 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
rcu_prepare_kthreads(cpu);
break;
case CPU_ONLINE:
- rcu_online_kthreads(cpu);
case CPU_DOWN_FAILED:
rcu_node_kthread_setaffinity(rnp, -1);
rcu_cpu_kthread_setrt(cpu, 1);
@@ -2410,6 +2401,7 @@ void __init rcu_init(void)
rcu_init_one(&rcu_sched_state, &rcu_sched_data);
rcu_init_one(&rcu_bh_state, &rcu_bh_data);
__rcu_init_preempt();
+ open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

/*
* We don't need protection against CPU-hotplug here because
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 7b9a08b..0fed6b9 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -439,6 +439,7 @@ static void rcu_preempt_offline_cpu(int cpu);
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
static void rcu_preempt_check_callbacks(int cpu);
static void rcu_preempt_process_callbacks(void);
+static void rcu_preempt_do_callbacks(void);
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_TREE_PREEMPT_RCU)
static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c8bff30..38d09c5 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -602,6 +602,11 @@ static void rcu_preempt_process_callbacks(void)
&__get_cpu_var(rcu_preempt_data));
}

+static void rcu_preempt_do_callbacks(void)
+{
+ rcu_do_batch(&rcu_preempt_state, &__get_cpu_var(rcu_preempt_data));
+}
+
/*
* Queue a preemptible-RCU callback for invocation after a grace period.
*/
@@ -997,6 +1002,10 @@ static void rcu_preempt_process_callbacks(void)
{
}

+static void rcu_preempt_do_callbacks(void)
+{
+}
+
/*
* Wait for an rcu-preempt grace period, but make it happen quickly.
* But because preemptible RCU does not exist, map to rcu-sched.
@@ -1299,15 +1308,10 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
raw_spin_unlock_irqrestore(&rnp->lock, flags);
sp.sched_priority = RCU_KTHREAD_PRIO;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+ wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
return 0;
}

-static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
-{
- if (rnp->boost_kthread_task)
- wake_up_process(rnp->boost_kthread_task);
-}
-
#else /* #ifdef CONFIG_RCU_BOOST */

static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
@@ -1331,10 +1335,6 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
return 0;
}

-static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
-{
-}
-
#endif /* #else #ifdef CONFIG_RCU_BOOST */

#ifndef CONFIG_SMP
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 1396017..40cf63d 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -58,7 +58,7 @@ DEFINE_PER_CPU(struct task_struct *, ksoftirqd);

char *softirq_to_name[NR_SOFTIRQS] = {
"HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
- "TASKLET", "SCHED", "HRTIMER"
+ "TASKLET", "SCHED", "HRTIMER", "RCU"
};

/*
diff --git a/tools/perf/util/trace-event-parse.c b/tools/perf/util/trace-event-parse.c
index 1e88485..0a7ed5b 100644
--- a/tools/perf/util/trace-event-parse.c
+++ b/tools/perf/util/trace-event-parse.c
@@ -2187,6 +2187,7 @@ static const struct flag flags[] = {
{ "TASKLET_SOFTIRQ", 6 },
{ "SCHED_SOFTIRQ", 7 },
{ "HRTIMER_SOFTIRQ", 8 },
+ { "RCU_SOFTIRQ", 9 },

{ "HRTIMER_NORESTART", 0 },
{ "HRTIMER_RESTART", 1 },
--
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