linux-kernel - Re: [RFC 2/2] perf: Marker software event and ioctl

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Date:	Tue, 16 Sep 2014 09:44:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Pawel Moll <pawel.moll@....com>,
	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [RFC 2/2] perf: Marker software event and ioctl


* David Ahern <dsahern@...il.com> wrote:

> On 9/12/14, 2:44 PM, Arnaldo Carvalho de Melo wrote:
> >Em Fri, Sep 12, 2014 at 10:37:39AM -0700, David Ahern escreveu:
> >>On 9/12/14, 4:48 AM, Pawel Moll wrote:
> >>>This patch adds a PERF_COUNT_SW_MARKER event type, which
> >>>can be requested by user and a PERF_EVENT_IOC_MARKER
> >>>ioctl command which will inject an event of said type into
> >>>the perf buffer. The ioctl can take a zero-terminated
> >>>string argument, similar to tracing_marker in ftrace,
> >>>which will be kept in the "raw" field of the sample.
> >>>
> >>>The main use case for this is synchronisation of
> >>>performance data generated in user space with the perf
> >>>stream coming from the kernel. For example, the marker
> >>>can be inserted by a JIT engine after it generated
> >>>portion of the code, but before the code is executed
> >>>for the first time, allowing the post-processor to
> >>>pick the correct debugging information. Other example
> >>>is a system profiling tool taking data from other
> >>>sources than just perf, which generates a marker
> >>>at the beginning at at the end of the session
> >>>(also possibly periodically during the session) to
> >>>synchronise kernel timestamps with clock values
> >>>obtained in userspace (gtod or raw_monotonic).
> >>
> >>Seems really similar to what I proposed in the past:
> >>
> >>https://lkml.org/lkml/2011/2/27/159
> >>
> >>Which was rejected.
> >
> >I took a look at that thread, but just barely, emphasis on that.
> >
> >Injecting something from userspace, a la ftrace, seems to be something,
> >as tglx mentioned, "buried" in that patchset.
> 
> Thomas object to an ioctl buried deep in a patch -- newbie 
> mistake.
> 
> Peter objected to the ioctl https://lkml.org/lkml/2011/3/1/229
> 
> It was not userspace injecting random data into the stream but 
> rather forcing the sample to be generated and added to the 
> stream.

I think adding an ioctl to inject user-provided data into the 
event stream is sensible, as long as there's a separate 'user 
generated data' event for it, etc.

The main usecase I could see would be to introduce a 
perf_printf() variant, supported by 'perf trace' by default, to 
add various tracable printouts to apps.

Timestamps generated by apps would be another usecase. It would 
probably be wise to add a 32-bit (or 64-bit) message type ID, 
plus a length field, with a message type registry somewhere in 
tools/perf/ (and reference implementation for each new subtype), 
to keep things organized yet flexible going forward.

Thanks,

	Ingo
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