Re: [patch 01/15] Kernel Tracepoints

From: Mathieu Desnoyers
Date: Tue Jul 15 2008 - 10:46:43 EST


* 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()
>
> ?
>

Yes, I would add them to include/linux/rcupdate.h. I'll include it with
my next release.

Talking about headers, I have noticed that placing headers with the code
may not be as clean as I would hope. For instance, the kernel/irq-trace.h
header, when included from kernel/irq/handle.c, has to be included with:

#include "../irq-trace.h"

Which is not _that_ bad, but we we want to instrument the irq handler
found in arch/x86/kernel/cpu/mcheck/mce_intel_64.c, including
#include "../../../../../kernel/irq-trace.h" makes me go "yeeeek!"

How about creating include/trace/irq.h and friends ?

Mathieu

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Mathieu Desnoyers
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