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Date: Fri, 27 Apr 2018 11:42:15 -0400 (EDT)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rostedt <rostedt@goodmis.org>
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Subject: Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks
 executing with preempt on
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----- On Apr 27, 2018, at 10:47 AM, rostedt rostedt@goodmis.org wrote:

> On Fri, 27 Apr 2018 10:26:29 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> The general approach and the implementation look fine, except for
>> one small detail: I would be tempted to explicitly disable preemption
>> around the call to the tracepoint callback for the rcuidle variant,
>> unless we plan to audit every tracer right away to remove any assumption
>> that preemption is disabled in the callback implementation.
> 
> I'm thinking that we do that audit. There shouldn't be many instances
> of it. I like the idea that a tracepoint callback gets called with
> preemption enabled.

I see that ftrace explicitly disables preemption in its ring buffer
code. FWIW, this is redundant when called from sched-rcu tracepoints
and from kprobes which adds unnecessary performance overhead.

LTTng expects preemption to be disabled when invoked. I can adapt on my
side as needed, but would prefer not to have redundant preemption disabling
for probes hooking on sched-rcu tracepoints (which is the common case).

Do perf callbacks expect preemption to be disabled ?

Thanks,

Mathieu

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