Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757989AbZDFUPh (ORCPT ); Mon, 6 Apr 2009 16:15:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753686AbZDFUP2 (ORCPT ); Mon, 6 Apr 2009 16:15:28 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:38535 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753038AbZDFUP1 (ORCPT ); Mon, 6 Apr 2009 16:15:27 -0400 Date: Mon, 6 Apr 2009 13:15:24 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Frederic Weisbecker , Tom Zanussi , Ingo Molnar , linux-kernel Subject: Re: [PATCH] tracing/filters: allow event filters to be set only when not tracing Message-ID: <20090406201524.GF6988@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1238830355.22495.55.camel@bookworm> <1238916865.7989.212.camel@tropicana> <20090405171146.GK6893@linux.vnet.ibm.com> <20090406161527.GD6988@linux.vnet.ibm.com> <20090406194417.GA5936@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5986 Lines: 153 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 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 --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/