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Date: Thu, 18 Dec 2014 15:20:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] perf ftrace: Factor regs retrieval for function tracer
Message-ID: <20141218142027.GE30905@twins.programming.kicks-ass.net>
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On Thu, Dec 18, 2014 at 03:08:47PM +0100, Jiri Olsa wrote:
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 6fa484de2ba1..54bffedd28b6 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -304,7 +304,7 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
>  {
>  	struct ftrace_entry *entry;
>  	struct hlist_head *head;
> -	struct pt_regs regs;
> +	struct pt_regs **regsp, *regs = pt_regs;
>  	int rctx;
>  
>  	head = this_cpu_ptr(event_function.perf_events);
> @@ -316,16 +316,24 @@ perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
>  
>  	BUILD_BUG_ON(ENTRY_SIZE > PERF_MAX_TRACE_SIZE);
>  
> -	perf_fetch_caller_regs(&regs);
> +	/*
> +	 * The ftrace_ops is registered with SAVE_REGS_IF_SUPPORTED,
> +	 * so if we got pt_regs defined, we don't need to retrieve
> +	 * regs buffer through perf_trace_buf_prepare.
> +	 */
> +	regsp = pt_regs ? NULL : &regs;
>  
> -	entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, NULL, &rctx);
> +	entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, regsp, &rctx);
>  	if (!entry)
>  		return;
>  
> +	if (regsp)
> +		perf_fetch_caller_regs(regs);
> +


Would not something like:

	struct pt_regs *regs;

	entry = perf_trace_buf_prepare(ENTRY_SIZE, TRACE_FN, &regs, &rctx);
	if (!pt_regs) {
		perf_fetch_caller_regs(regs);
		pt_regs = regs;
	}

Be simpler?