linux-kernel - Re: [PATCH v2 2/5]: trace_event: export HZ in timer's tracepoint format

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Date:	Mon, 28 Dec 2009 18:40:13 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/5]: trace_event: export HZ in timer's tracepoint
 format



Ingo Molnar wrote:

> 
> 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?
> 

Sure, this is the better way, i'll fix it address your suggestion in the next version.

Thanks,
Xiao
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