linux-kernel - Re: [PATCH 5/6] tracing: reduce latency and remove percpu trace

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Date:	Wed, 20 Jan 2010 20:22:27 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 5/6] tracing: reduce latency and remove percpu trace_seq

On Tue, Jan 19, 2010 at 03:34:22PM +0800, Lai Jiangshan wrote:
> 
> __print_flags() and __print_symbolic() use percpu trace_seq:
> 
> 1) Its memory is preallocated, it wastes memory when we don't use tracing.
> 2) It wastes memory for multi-cpus system.
> 3) It disables preemption when it executes its core routine
>    "trace_seq_printf(s, "%s: ", #call);" and introduce latency
>    for more important process.
> 
> So we move this trace_seq to struct trace_iterator.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index be9ece5..348500d 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -12,9 +12,6 @@ struct dentry;
>  
>  #define FTRACE_SEQ_BUFSIZE PAGE_SIZE
>  
> -DECLARE_PER_CPU(struct trace_seq, ftrace_event_seq);
> -DECLARE_PER_CPU(unsigned char[FTRACE_SEQ_BUFSIZE], ftrace_event_buffer);
> -
>  struct trace_print_flags {
>  	unsigned long		mask;
>  	const char		*name;
> @@ -60,6 +57,10 @@ struct trace_iterator {
>  	struct trace_seq	seq;
>  	unsigned char		buffer[FTRACE_SEQ_BUFSIZE];
>  
> +	/* trace_seq for __print_flags() and __print_symbolic() */
> +	struct trace_seq	tmp_seq;
> +	unsigned char 		tmp_buffer[FTRACE_SEQ_BUFSIZE];




Well, I don't like much that because it's a temporary buffer
in trace iter only used by few events.
But the problem is indeed tricky.

May be should we use a kmalloc in raw_output?

We could pass a trace_seq without buffer in ftrace_print_flags_seq
which can alloc the buffer and then free it after?

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