linux-kernel - Re: [PATCH 3/3] tracing/perf: Avoid perf_trace_buf_*() in perf_trace

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Date:	Wed, 19 Jun 2013 20:12:11 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] tracing/perf: Avoid perf_trace_buf_*() in
	perf_trace_##call() when possible

On 06/18, Steven Rostedt wrote:
>
> On Tue, 2013-06-18 at 21:22 +0200, Oleg Nesterov wrote:
> > @@ -663,6 +663,12 @@ perf_trace_##call(void *__data, proto)					\
> >  	int rctx;							\
> >  									\
> >  	__data_size = ftrace_get_offsets_##call(&__data_offsets, args); \
> > +									\
> > +	head = this_cpu_ptr(event_call->perf_events);			\
> > +	if (__builtin_constant_p(!__task) && !__task &&			\
>
>
> I'm trying to wrap my head around this:
>
>   __builtin_constant_p(!task)
>
> is this the same as:
>
>   !__builtin_constant_p(task)
>
> Or is it the same as:
>
>   __builtin_constant_p(task)
>
> ?
>
> Because that '!' is confusing the heck out of me.
>
> If !task is a constant, wouldn't task be a constant too, and if task is
> not a constant then I would also assume !task is not a constant as well.

!__task looks more explicit/symmetrical to me. We need

	if (is_compile_time_true(!__task)) && list_empty)
		return;

is_compile_time_true(cond) could be defined as

	__builtin_constant_p(cond) && (cond)
or
	__builtin_constant_p(!cond) && (cond)

but the 1ts one looks more clean.

However,

> If this is the case, can we nuke the '!' from the builtin_consant_p().

OK, I do not really mind, will do.

And,

> Or is this your way to confuse me as much as my code has confused
> you? ;-)

Of course! this was the main reason.


Steven, I convinced myself the patch should be correct. If you agree with
this hack:

	- anything else I should do apart from the change above?

	- should I resend the previous "[PATCH 0/3] tracing: more
	  list_empty(perf_events) checks" series?

	  This series depends on "[PATCH 3/3] tracing/perf: Move the
	  PERF_MAX_TRACE_SIZE check into perf_trace_buf_prepare()".

	  Or I can drop this patch if you do not like it and rediff.

	  Just in case, there are other pending patches in trace_kprobe.c
	  which I am going to resend, but they are orthogonal.

Oleg.

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