linux-kernel - Re: [PATCH 7/9] ftrace, perf: Add support to use function tracepoint in perf

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Date:	Mon, 28 Nov 2011 21:08:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, fweisbec@...il.com, mingo@...hat.com,
	paulus@...ba.org, acme@...stprotocols.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9] ftrace, perf: Add support to use function
 tracepoint in perf

On Mon, 2011-11-28 at 14:58 -0500, Steven Rostedt wrote:
> > +static int perf_ftrace_function_register(struct perf_event *event)
> > +{
> > +     struct ftrace_ops *ops = &event->ftrace_ops;
> > +
> > +     ops->flags |= FTRACE_OPS_FL_CONTROL;
> > +     atomic_set(&ops->disabled, 1);
> > +     ops->func = perf_ftrace_function_call;
> > +     return register_ftrace_function(ops);
> 
> When is ADD called? Because as soon as you register this function, even
> though you have it "disabled" the system takes about a 13% impact on
> performance just by calling this.

Typically at context switch time.

> > +static void perf_ftrace_function_enable(struct perf_event *event)
> > +{
> > +     struct ftrace_ops *ops = &event->ftrace_ops;
> > +     enable_ftrace_function(ops);
> 
> Is it really an issue that we shouldn't call the full blown register
> instead? I'm not really understanding why this is a problem. Note, one
> of the improvements to ftrace in the near future is to enable ftrace
> without stop_machine. 

Yeah, but still not something you want to do during context switches,
right?
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