Messages in this thread | | | Date | Fri, 27 Apr 2018 12:07:54 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on |
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On Fri, 27 Apr 2018 11:42:15 -0400 (EDT) Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> ----- 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.
Sure, but that code is called from other locations that do not have preemption disabled. Calling preempt_disable() is far from the biggest overhead of that code path.
> > 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).
Why not? Really, preempt_disable is simply a per cpu counter, with only need of adding compiler barriers.
> > Do perf callbacks expect preemption to be disabled ?
I'll have to look, but wouldn't be hard to change.
-- Steve
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