linux-kernel - Re: [PATCH] tracing: Export key trace event symbols

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Message-ID: <1429802909.3242.23.camel@arm.com>
Date:	Thu, 23 Apr 2015 16:28:29 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	David Ahern <dsahern@...il.com>
Cc:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ron Rechenmacher <ron@...l.gov>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] tracing: Export key trace event symbols

On Wed, 2015-04-22 at 16:36 +0100, David Ahern wrote:
> On 4/22/15 8:47 AM, Arnaldo Carvalho de Melo wrote:
> > Em Wed, Apr 22, 2015 at 08:53:14AM -0400, Steven Rostedt escreveu:
> >> >On Tue, 21 Apr 2015 21:24:51 -0500
> >> >Ron Rechenmacher<ron@...l.gov>  wrote:
> >>> > >I've looked at the above reference briefly and it appears that user-space
> >>> > >would be mmapping the buffer read-only. Is that correct?
> >> >
> >> >Correct, but I'm sure we could still add something (if it doesn't
> >> >already exist) to have userspace write into the buffer. Ftrace has that
> >> >with the trace_marker file.
> > There is something in the works, I guess Pawell Moll (sp) was working on it, and
> > David Ahern (CCed) should know, David?
> >
> 
> I played around with generating perf events in userspace with the 
> intention of having the userspace events get merged with kernel events 
> during the processing stage, but I did not take it to the point of 
> integrating into perf. This was around October 2013. I got distracted 
> with other topics and have not come back to it.
> 
> Pawel has a patch that allows userspace to inject events into the stream 
> via ioctl calls.

In the last version it was even a prctl - no need for a perf file
descriptor any more :-)

But the patch requires more care if it's to go in, so I'm open to people
screaming "yes, we need it!" ;-)

> Stephane also injects events for JIT.

But that, as far as I understand, happens in userspace (as in: nothing
goes down to kernel)?

> One of the key requirements is a common time basis (e.g., 
> CLOCK_MONOTONIC or PERF_CLOCK) to be able to merge the events properly. 
> I have a kernel module that exports perf_clock to userspace via 
> clock_gettime; the 4.1 kernel should have the code that allows the clock 
> id to be specified providing a solution to this problem.

It's in! :-) After all these years...

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34f439278cef7b1177f8ce24f9fc81dfc6221d3b

Pawel

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