Re: [patch 01/15] Kernel Tracepoints

[Mathieu Desnoyers]

* Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> On Tue, 2008-07-15 at 09:25 -0400, Mathieu Desnoyers wrote:
> > * Peter Zijlstra (peterz@xxxxxxxxxxxxx) 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()
> 
> ?
> 

Actually I think it's better to call them
rcu_read_lock_sched() and rcu_read_unlock_sched() to match the _bh()
equivalent already in rcupdate.h.

Mathieu


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Mathieu Desnoyers
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