linux-kernel - Re: perf/ftrace: does PERF_EVENT_IOC_SET

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Date:	Tue, 9 Jun 2015 16:18:16 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	David Ahern <dsahern@...il.com>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: perf/ftrace: does PERF_EVENT_IOC_SET_FILTER serve any purpose?

On Tue, 9 Jun 2015, David Ahern wrote:

> On 6/9/15 2:02 PM, Vince Weaver wrote:
> > # ls/sys/kernel/debug/tracing/events/irq/irq_handler_entry/
> > enable	filter	format	id  trigger
> > 
> > # perf list
> > ....
> >    [ Tracepoints not available: No such file or directory ]
> > 
> > strace shows
> > statfs("/sys/kernel/debug/tracing/events", {f_type=0x74726163,
> > f_bsize=4096, f_blocks=0, f_bfree=0, f_bavail=0, f_files=0, f_ffree=0,
> > f_fsid={0, 0}, f_namelen=255, f_frsize=4096}) = 0
> > 
> > so I have no idea what's going wrong here.
> 
> Check Steven's email; probably the issue.
> 
> > 
> > 
> > Anyway, on a related question, I see for the event that you use
> > 	--filter irq==28
> > How do I find out the valid filters for each event?  Do I have to parse
> > the "format" file under
> > 	/sys/kernel/debug/tracing/events/*/* ?
> 
> Yes. And *very important* don't use the print fmt names, but the field names.
> Often they are the same but when they differ it can be maddening.

OK, thanks, this was very helpful, combined with figuring out the perf 
issue, I finally have some code that is successfully using the ioctl.

The ftrace documentation says you can clear a filter by writing "0" to it, 
but that doesn't seem to work with the ioctl.

Is there better documentation for the filters than the ftrace.txt file 
that comes with the kernel?

Vince
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