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Date:   Mon, 23 Apr 2018 13:12:21 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you
 can
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Thread-Topic: irqflags: Avoid unnecessary calls to trace_ if you can
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----- On Apr 23, 2018, at 12:18 PM, rostedt rostedt@goodmis.org wrote:

> On Mon, 23 Apr 2018 10:59:43 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> The main open question here is whether we want one SRCU grace period
>> domain per SRCU tracepoint definition, or just one SRCU domain for all
>> SRCU tracepoints would be fine.
>> 
>> I'm not sure what we would gain by having the extra granularity provided
>> by one SRCU grace period domain per tracepoint, and having a single SRCU
>> domain for all SRCU tracepoints makes it easy to batch grace period after
>> bulk tracepoint modifications.
>> 
>> Thoughts ?
> 
> I didn't think too much depth in this. It was more of just a brain
> storming idea. Yeah, one singe RCU domain may be good enough. I was
> thinking more of how to know when a tracepoint required the SRCU domain
> as supposed to a preempt disabled domain, and wanted to just suggest
> the linker script approach.
> 
> This is how I detect if trace_printk() is used anywhere in the kernel
> (and do that big warning if it is). That way the trace events don't
> need to be created any special way. You just use the trace_##event##_X
> flavor and it automatically detects what to do. But we need to make
> sure the same event isn't used for multiple flavors (SRCU vs schedule),
> or maybe we can, and any change would have to do both synchronizations.

The approach I used for synchronize rcu a few years ago when I did a srcu
tracepoint prototype [1] was simply this:

 static inline void tracepoint_synchronize_unregister(void)
 {
+	synchronize_srcu(&tracepoint_srcu);
 	synchronize_sched();
 }

So whenever we synchronize after tracepoint unregistration, the tracepoint
code always issue both synchronize_sched() and SRCU synchronize. This way,
tracepoint API users don't have to care about the kind of tracepoint they
are updating.

I'm inclined to explicitly declare the tracepoints with their given
synchronization method. Tracepoint probe callback functions for currently
existing tracepoints expect to have preemption disabled when invoked.
This assumption will not be true anymore for srcu-tracepoints.

Thanks,

Mathieu

[1] https://github.com/compudj/linux-dev/commits/tracepoint-srcu

> 
> -- Steve

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com