Date: Mon, 28 Dec 2009 08:54:17 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Steven Rostedt <rostedt@goodmis.org>, Paul Mackerras <paulus@samba.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/5]: trace_event: export HZ in timer's tracepoint
 format
Message-ID: <20091228075417.GB20039@elte.hu>
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* Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> wrote:

> Export HZ in timer's tracepoint, we can use it to get
> TIMER/ITIMER_VIRTUAL/ITIMER_PROF exact latency and it's
> suggested by Ingo
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
>  include/trace/events/timer.h |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/include/trace/events/timer.h b/include/trace/events/timer.h
> index 13ec15a..7749ae5 100644
> --- a/include/trace/events/timer.h
> +++ b/include/trace/events/timer.h
> @@ -74,14 +74,17 @@ TRACE_EVENT(timer_expire_entry,
>  	TP_STRUCT__entry(
>  		__field( void *,	timer	)
>  		__field( unsigned long,	now	)
> +		__field( int,		hz	)
>  	),
>  
>  	TP_fast_assign(
>  		__entry->timer		= timer;
>  		__entry->now		= jiffies;
> +		__entry->hz		= HZ;
>  	),
>  
> -	TP_printk("timer=%p now=%lu", __entry->timer, __entry->now)
> +	TP_printk("timer=%p now=%lu HZ=%d", __entry->timer, __entry->now,
> +		  __entry->hz)
>  );

I think we can do something slightly different and more efficient: just create 
a new timer event to report the value of HZ.

That way we dont clutter the timer_expire_entry record format with a 
repetitive HZ field. It's an extra 4 bytes overhead: that has to be written, 
passed along, copied and thrown away in 99.9999% of the cases - such overhead 
should be avoided.

If you created a special timer_params event, which would produce precisely one 
event when triggered via say a new perf ioctl. I.e. add something like this to 
perf_event.h:

  #define PERF_EVENT_IOC_INJECT       _IOW('$', 7, __u64)

and add code to kernel/perf_event.c's perf_ioctl() function that takes that 
__u64 parameter as an event ID and injects an 'artificial' event.

Such a new feature would be useful for other things as well: backtesting rare 
events, injecting other types of 'parameter/query events', etc.

There might be more details to this, but it would be a useful scheme IMO - and 
it would still integrate nicely with the whole ftrace event enumeration scheme 
so tooling support would be easier.

What do you think?

	Ingo
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