linux-kernel - Re: [patch 01/15] Kernel Tracepoints

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Message-Id: <1216130593.12595.189.camel@twins>
Date:	Tue, 15 Jul 2008 16:03:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <mhiramat@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Hideo AOKI <haoki@...hat.com>,
	Takashi Nishiie <t-nishiie@...css.fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [patch 01/15] Kernel Tracepoints

On Tue, 2008-07-15 at 09:25 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Wed, 2008-07-09 at 10:59 -0400, Mathieu Desnoyers wrote:

> > > +#define __DO_TRACE(tp, proto, args)					\
> > > +	do {								\
> > > +		int i;							\
> > > +		void **funcs;						\
> > > +		preempt_disable();					\
> > > +		funcs = (tp)->funcs;					\
> > > +		smp_read_barrier_depends();				\
> > > +		if (funcs) {						\
> > > +			for (i = 0; funcs[i]; i++) {			\
> > 
> > Also, why is the preempt_disable needed?
> > 
> 
> Addition and removal of tracepoints is synchronized by RCU using the
> scheduler (and preempt_disable) as guarantees to find a quiescent state
> (this is really RCU "classic"). The update side uses rcu_barrier_sched()
> with call_rcu_sched() and the read/execute side uses
> "preempt_disable()/preempt_enable()".

> > > +static void tracepoint_entry_free_old(struct tracepoint_entry *entry, void *old)
> > > +{
> > > +	if (!old)
> > > +		return;
> > > +	entry->oldptr = old;
> > > +	entry->rcu_pending = 1;
> > > +	/* write rcu_pending before calling the RCU callback */
> > > +	smp_wmb();
> > > +#ifdef CONFIG_PREEMPT_RCU
> > > +	synchronize_sched();	/* Until we have the call_rcu_sched() */
> > > +#endif
> > 
> > Does this have something to do with the preempt_disable above?
> > 
> 
> Yes, it does. We make sure the previous array containing probes, which
> has been scheduled for deletion by the rcu callback, is indeed freed
> before we proceed to the next update. It therefore limits the rate of
> modification of a single tracepoint to one update per RCU period. The
> objective here is to permit fast batch add/removal of probes on
> _different_ tracepoints.
> 
> This use of "synchronize_sched()" can be changed for call_rcu_sched() in
> linux-next, I'll fix this.

Right, I thought as much, its just that the raw preempt_disable()
without comments leaves one wondering if there is anything else going
on.

Would it make sense to add:

rcu_read_sched_lock()
rcu_read_sched_unlock()

to match:

call_rcu_sched()
rcu_barrier_sched()
synchronize_sched()

?

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