linux-kernel - Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock

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Date:	Thu, 5 Mar 2009 12:04:33 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing/function-graph-tracer: use the more
	lightweight local clock


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> Impact: decrease hangs risks with the graph tracer on slow systems
> 
> Since the function graph tracer can spend too much time on 
> timer interrupts, it's better now to use the more lightweight 
> local clock. Anyway, the function graph traces are more 
> reliable on a per cpu trace.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  arch/x86/kernel/ftrace.c             |    2 +-
>  kernel/trace/trace_functions_graph.c |    2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 3925ec0..40960c2 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -436,7 +436,7 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
>  		return;
>  	}
>  
> -	calltime = cpu_clock(raw_smp_processor_id());
> +	calltime = sched_clock();

I changed this to trace_clock_local() :-)

(btw, we should turn trace_clock_local() into an inline 
function)

There will be one problem though: function trace elapsed time 
measurements across idle. Those are not correctly measured by 
the local clock.

	Ingo
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