Messages in this thread | | | Date | Thu, 26 Jan 2012 03:37:29 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/7] ftrace: Add enable/disable ftrace_ops control interface |
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On Wed, Jan 25, 2012 at 06:13:41PM -0500, Steven Rostedt wrote: > On Fri, 2012-01-20 at 18:02 +0100, Frederic Weisbecker wrote: > > > > > +/** > > > + * ftrace_function_enable - enable controlled ftrace_ops on given cpu > > > + * > > > + * This function enables tracing on given cpu by decreasing > > > + * the per cpu control variable. > > > + * It must be called with preemption disabled and only on > > > + * ftrace_ops registered with FTRACE_OPS_FL_CONTROL. > > > + */ > > > +static inline void ftrace_function_enable(struct ftrace_ops *ops, int cpu) > > > +{ > > > + atomic_t *disabled; > > > + > > > + if (WARN_ON_ONCE(!(ops->flags & FTRACE_OPS_FL_CONTROL) || > > > + !preempt_count())) > > > + return; > > > + > > > + disabled = per_cpu_ptr(ops->disabled, cpu); > > > + atomic_dec(disabled); > > > +} > > > > As you're using this for the local CPU exclusively, I suggest you rather > > rename it to "ftrace_function_{dis,en}able_cpu(struct ftrace_ops *ops)" > > I wonder if "ftrace_function_local_{dis,en}able(ops)" would be better? > That would match something like local_irq_disable/enable.
Good idea.
> > > and use __get_cpu_var() that does the preempt check for you. > > Hmm, I haven't tried that with allocated per_cpu pointers before. If it > works, sure.
Yeah that works. In fact this_cpu_ptr() would do the best. Both this_cpu_ptr() and __get_cpu_var() map to the same internal ops with preemption checks.
> > > static void > > > +ftrace_ops_control_func(unsigned long ip, unsigned long parent_ip) > > > +{ > > > + struct ftrace_ops *op; > > > + int cpu; > > > + > > > + if (unlikely(trace_recursion_test(TRACE_CONTROL_BIT))) > > > + return; > > > + > > > + /* > > > + * Some of the ops may be dynamically allocated, > > > + * they must be freed after a synchronize_sched(). > > > + */ > > > + preempt_disable_notrace(); > > > + trace_recursion_set(TRACE_CONTROL_BIT); > > > + cpu = smp_processor_id(); > > > + op = rcu_dereference_raw(ftrace_control_list); > > > + while (op != &ftrace_list_end) { > > > + if (!control_ops_is_disabled(op, cpu) && > > > + ftrace_ops_test(op, ip)) > > > + op->func(ip, parent_ip); > > > + > > > + op = rcu_dereference_raw(op->next); > > > > Should it be rcu_dereference_sched() ? > > From the comment posted by Paul McKenney who converted the global_list > ops (that does somewhat the same thing as the control ops here): > > /* > * Traverse the ftrace_global_list, invoking all entries. The reason that we > * can use rcu_dereference_raw() is that elements removed from this list > * are simply leaked, so there is no need to interact with a grace-period > * mechanism. The rcu_dereference_raw() calls are needed to handle > * concurrent insertions into the ftrace_global_list. > * > * Silly Alpha and silly pointer-speculation compiler optimizations! > */
Ok.
> > > But then reading the commit he has: > > Replace the calls to read_barrier_depends() in > ftrace_list_func() with rcu_dereference_raw() to improve > readability. The reason that we use rcu_dereference_raw() here > is that removed entries are never freed, instead they are simply > leaked. This is one of a very few cases where use of > rcu_dereference_raw() is the long-term right answer. And I > don't yet know of any others. ;-) > > Hmm, and I use the rcu_derefrence_raw() in other places in this file, > but those places now get freed. Although, I'm a bit nervous in changing > these to rcu_dereference_sched, because if CONFIG_DEBUG_LOCK_ALLOC is > enabled, then the checks will be done for *every function* called. > > We obviously have preemption disabled, or other bad things may happen. > Wonder if we really need this? Ftrace itself is a internal checker and > not truly a kernel component. It may be "exempt" from theses checks ;-) > > I could make the switch and see what overhead this causes. It may live > lock the system. It wouldn't be the first time lockdep & ftrace live > locked the system. Or made it so unbearably slow.
Yeah right. We can probably make this exception for function tracing by keeping rcu_dereference_raw(). But now that it involves a grace period, the comment probably should be updated :)
> Lockdep and ftrace do > not play well together. They both are very intrusive. The two remind me > of the United States congress. Where there is two parties trying to take > control of everything, but nothing ever gets done. We end up with a > grid/live lock in the country/computer.
:o)
> > > > > + }; > > > + trace_recursion_clear(TRACE_CONTROL_BIT); > > > + preempt_enable_notrace(); > > > +} > > > + > > > +static struct ftrace_ops control_ops = { > > > + .func = ftrace_ops_control_func, > > > +}; > > > > So note this patch is optimizing for the off case (when > > we have called pmu->del()), but at the cost of having an > > impact in the on case with having at least one level > > of multiplexing (and two on the worst case if we have ftrace > > running in parallel but this is enough a corner case that we > > don't care). > > > > But this is perhaps still a win. > > There's a lot more overhead elsewhere that this shouldn't be an issue.
Yeah that looks fine.
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