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

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Date:	Thu, 03 Jun 2010 09:36:10 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Theodore Tso <tedtso@...il.com>
Subject: Re: [PATCH] tracing: reduce latency and remove percpu trace_seq

On Thu, 2010-06-03 at 18:26 +0800, Lai Jiangshan wrote:
> __print_flags() and __print_symbolic() use percpu trace_seq:
> 
> 1) Its memory is allocated at compile time, it wastes memory if we don't use tracing.
> 2) It is percpu data and it wastes more memory for multi-cpus system.
> 3) It disables preemption when it executes its core routine
>    "trace_seq_printf(s, "%s: ", #call);" and introduces latency.
> 
> So we move this trace_seq to struct trace_iterator.

I like the patch, except for one thing.

[ added Ted because he was the one needing this ]

> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
>  include/linux/ftrace_event.h |    5 +++--
>  include/trace/ftrace.h       |   12 +++---------
>  kernel/trace/trace_output.c  |    3 ---
>  3 files changed, 6 insertions(+), 14 deletions(-)
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index 3167f2d..25e264d 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -11,8 +11,6 @@ struct trace_array;
>  struct tracer;
>  struct dentry;
>  
> -DECLARE_PER_CPU(struct trace_seq, ftrace_event_seq);
> -
>  struct trace_print_flags {
>  	unsigned long		mask;
>  	const char		*name;
> @@ -58,6 +56,9 @@ struct trace_iterator {
>  	struct ring_buffer_iter	*buffer_iter[NR_CPUS];
>  	unsigned long		iter_flags;
>  
> +	/* trace_seq for __print_flags() and __print_symbolic() etc. */
> +	struct trace_seq	tmp_seq;
> +
>  	/* The below is zeroed out in pipe_read */
>  	struct trace_seq	seq;
>  	struct trace_entry	*ent;
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 5a64905..19369db 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -145,7 +145,7 @@
>   *	struct trace_seq *s = &iter->seq;
>   *	struct ftrace_raw_<call> *field; <-- defined in stage 1
>   *	struct trace_entry *entry;
> - *	struct trace_seq *p;
> + *	struct trace_seq *p = &iter->tmp_seq;
>   *	int ret;
>   *
>   *	entry = iter->ent;
> @@ -157,12 +157,10 @@
>   *
>   *	field = (typeof(field))entry;
>   *
> - *	p = &get_cpu_var(ftrace_event_seq);
>   *	trace_seq_init(p);
>   *	ret = trace_seq_printf(s, "%s: ", <call>);
>   *	if (ret)
>   *		ret = trace_seq_printf(s, <TP_printk> "\n");
> - *	put_cpu();
>   *	if (!ret)
>   *		return TRACE_TYPE_PARTIAL_LINE;
>   *
> @@ -216,7 +214,7 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags,	\
>  	struct trace_seq *s = &iter->seq;				\
>  	struct ftrace_raw_##call *field;				\
>  	struct trace_entry *entry;					\
> -	struct trace_seq *p;						\
> +	struct trace_seq *p = &iter->tmp_seq;				\
>  	int ret;							\
>  									\
>  	event = container_of(trace_event, struct ftrace_event_call,	\
> @@ -231,12 +229,10 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags,	\
>  									\
>  	field = (typeof(field))entry;					\
>  									\
> -	p = &get_cpu_var(ftrace_event_seq);				\
>  	trace_seq_init(p);						\
>  	ret = trace_seq_printf(s, "%s: ", event->name);			\
>  	if (ret)							\
>  		ret = trace_seq_printf(s, print);			\
> -	put_cpu();							\

The "print" in the trace_seq_printf() is the TP_printk() field of the
TRACE_EVENT. I believe that there were some events (ext4?) that did
things that required preemption disabled.

Ted,

Is this still the case?

-- Steve

>  	if (!ret)							\
>  		return TRACE_TYPE_PARTIAL_LINE;				\
>  									\
> @@ -255,7 +251,7 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags,	\
>  	struct trace_seq *s = &iter->seq;				\
>  	struct ftrace_raw_##template *field;				\
>  	struct trace_entry *entry;					\
> -	struct trace_seq *p;						\
> +	struct trace_seq *p = &iter->tmp_seq;				\
>  	int ret;							\
>  									\
>  	entry = iter->ent;						\
> @@ -267,12 +263,10 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags,	\
>  									\
>  	field = (typeof(field))entry;					\
>  									\
> -	p = &get_cpu_var(ftrace_event_seq);				\
>  	trace_seq_init(p);						\
>  	ret = trace_seq_printf(s, "%s: ", #call);			\
>  	if (ret)							\
>  		ret = trace_seq_printf(s, print);			\
> -	put_cpu();							\
>  	if (!ret)							\
>  		return TRACE_TYPE_PARTIAL_LINE;				\
>  									\
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index 57c1b45..1ba64d3 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -16,9 +16,6 @@
>  
>  DECLARE_RWSEM(trace_event_mutex);
>  
> -DEFINE_PER_CPU(struct trace_seq, ftrace_event_seq);
> -EXPORT_PER_CPU_SYMBOL(ftrace_event_seq);
> -
>  static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly;
>  
>  static int next_event_type = __TRACE_LAST_TYPE + 1;
> 


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