linux-kernel - Re: [RFC] tracing: adding flags to events

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Message-ID: <4A0CD0F1.5020208@cn.fujitsu.com>
Date:	Fri, 15 May 2009 10:18:25 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] tracing: adding flags to events

Steven Rostedt wrote:
> Christoph has been asking about processing flags in the output. He rather 
> not see c2, and rather see what those three bits are. This patch is 
> an RFC to do just that. To test it out, I added the previous task state to 
> sched switch and used the flag processing to the printk of the 
> sched_switch event.
> 
> 
> To add a flag, just add __print_flags to the TP_printk arguments.
> 
> 	TP_STRUCT__entry(
> 		__field(	unsigned int,	flags		)
> 	),
> 
> 	TP_printk("flags are %s", __print_flags(__entry->flags,
> 			0, "BIT0", 1, "BIT1", 2, "BIT2", -1))
> 
> 
> Thus __print_flags prototype would look like:
> 
> const char *__print_flags(long flags, ...);
> 

How about __print_flags(long flags, char *delim, ...); ?

Take file mode for example, the output will be "rwx", but not "r|w|x".

> But it is actually converted to other helper functions to handle the 
> string. The trick that ftrace does, is disables preemption before calling 
> the printk, uses a percpu buffer, and passes that in to a helper function 
> that will print out the flags.
> 
>  You could see "flags are BIT1|BIT2"  if flags was 6 in the above case.
> 


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