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Message-ID: <20090406201524.GF6988@linux.vnet.ibm.com>
Date: Mon, 6 Apr 2009 13:15:24 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Tom Zanussi <tzanussi@...il.com>, Ingo Molnar <mingo@...e.hu>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/filters: allow event filters to be set only
when not tracing
On Mon, Apr 06, 2009 at 03:52:55PM -0400, Steven Rostedt wrote:
>
> On Mon, 6 Apr 2009, Frederic Weisbecker wrote:
> > > >
> > > > > > > So assuming we can't use rcu for this, it would be nice to have a way to
> > > > > > > 'pause' tracing so the current filter can be removed i.e. some version
> > > > > > > of stop_trace()/start_trace() that make sure nothing is still executing
> > > > > > > or can enter filter_match_preds() while the current call->preds is being
> > > > > > > destroyed. Seems like it would be straightforward to implement for the
> > > > > > > event tracer, since each event maps to a tracepoint that could be
> > > > > > > temporarily unregistered/reregistered, but maybe not so easy for the
> > > > > > > ftrace tracers...
> > > > > >
> > > > > > In principle, it would be possible to rework RCU so that instead of the
> > > > > > whole idle loop being a quiescent state, there is a single quiescent state
> > > > > > at one point in each idle loop. The reason that I have been avoiding this
> > > > > > is that there are a lot of idle loops out there, and it would be a bit
> > > > > > annoying to (1) find them all and update them and (2) keep track of all of
> > > > > > them to ensure that new ones cannot slip in without the quiescent state.
> > > > > >
> > > > > > But it could be done if the need is there. Simple enough change.
> > > > > > The following patch shows the general approach, assuming that CPUs
> > > > > > are never put to sleep without entering nohz mode.
> > > > > >
> > > > > > Thoughts?
> > > > >
> > > > > I think using synchronize_sched() should be good enough for what we need.
> > > >
> > > > Again, as long as either (1) you are OK with synchronize_sched()
> > > > ignoring preempt-disable sequences in the idle loop or (2) we rework RCU
> > > > to add something like an rcu_idle() call in each idle loop.
> > >
> > > 3) add "notrace" to the idle functions ;-)
> > >
> > > But perhaps the rcu_idle might be the best idea.
> >
> >
> > And tracing the idle time is also sometimes very useful :-)
>
> Agreed. I guess choice 2 is the best answer.
Fair enough!
Would one of you please check the placement of the rcu_idle() in the
patch? Patch reproduced below for convenience.
Thanx, Paul
------------------------------------------------------------------------
>From 7e08c37b20cb3d93ba67f8ad5d46f2c38acb8fe5 Mon Sep 17 00:00:00 2001
From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date: Sun, 5 Apr 2009 10:09:54 -0700
Subject: [PATCH] Make idle loop (mostly) safe for RCU read-side critical sections.
Not for inclusion, demo only. Untested, probably fails to compile.
This patch is for demonstration purposes only. It adds a facility to
rcutree.c to allow RCU read-side critical sections to be used in
idle loops, as long as those RCU read-side critical sections do not
lap over the call to rcu_idle().
If this were a real patch, it would have the following:
o A config variable to allow architectures to opt out of this
sort of behavior. (But then again, maybe not.)
o Follow-up patches that added a call to rcu_idle() to each
idle loop in the kernel, probably grouped by architecture.
o Documentation updates to explain the new loosened restrictions
regarding RCU read-side critical sections and idle loops.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
arch/x86/kernel/process.c | 1 +
include/linux/rcupdate.h | 1 +
kernel/rcutree.c | 21 ++++++++++++++-------
3 files changed, 16 insertions(+), 7 deletions(-)
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 156f875..adbaf13 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -310,6 +310,7 @@ void default_idle(void)
current_thread_info()->status |= TS_POLLING;
trace_power_end(&it);
} else {
+ rcu_idle();
local_irq_enable();
/* loop is done by the caller */
cpu_relax();
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 528343e..3905f54 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -265,6 +265,7 @@ extern void synchronize_rcu(void);
extern void rcu_barrier(void);
extern void rcu_barrier_bh(void);
extern void rcu_barrier_sched(void);
+extern void rcu_idle(void);
/* Internal to kernel */
extern void rcu_init(void);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 97ce315..4c61b71 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -937,6 +937,17 @@ static void rcu_do_batch(struct rcu_data *rdp)
}
/*
+ * Called from each idle loop to enable RCU to treat the idle loop as
+ * a quiescent state. Note that this code assumes that idle CPUs continue
+ * executing instructions until they enter nohz mode.
+ */
+void rcu_idle(void)
+{
+ rcu_qsctr_inc(cpu);
+ rcu_bh_qsctr_inc(cpu);
+}
+
+/*
* Check to see if this CPU is in a non-context-switch quiescent state
* (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
* Also schedule the RCU softirq handler.
@@ -947,15 +958,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
*/
void rcu_check_callbacks(int cpu, int user)
{
- if (user ||
- (idle_cpu(cpu) && rcu_scheduler_active &&
- !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+ if (user) {
/*
- * Get here if this CPU took its interrupt from user
- * mode or from the idle loop, and if this is not a
- * nested interrupt. In this case, the CPU is in
- * a quiescent state, so count it.
+ * Get here if this CPU took its interrupt from user mode.
+ * In this case, the CPU is in a quiescent state, so count it.
*
* No memory barrier is required here because both
* rcu_qsctr_inc() and rcu_bh_qsctr_inc() reference
--
1.5.2.5
--
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