Re: [PATCH v2 2/2] perf: Userspace event

From: Ingo Molnar
Date: Wed Sep 24 2014 - 03:20:27 EST

* Namhyung Kim <namhyung@xxxxxxxxxx> wrote:

> On Tue, 23 Sep 2014 18:03:07 +0100, Pawel Moll wrote:
> > This patch adds a new PERF_COUNT_SW_UEVENT software event
> > and a related PERF_SAMPLE_UEVENT sample. User can now
> > write to the the perf file descriptor, injecting such
> > event in the perf buffer.
> It seems the PERF_SAMPLE_UEVENT sample can be injected to any event. So
> why the PERF_COUNT_SW_UEVENT is needed? At least one can use the
> SW_DUMMY event for that purpose.
> Also I think it'd be better to be a record type (PERF_RECORD_XXX)
> instead of a sample flag (PERF_SAMPLE_XXX). In perf tools, we already
> use perf_user_event_type for synthesized userspace events. This way it
> can avoid unnecessary sample processing for userspace events.
> For contents, I prefer to give complete control to users - kernel
> doesn't need to care about it other than its size. If one just wants to
> use strings only, she can write them directly. If others want to mix
> different types of data, they might need to define a data format for
> their use.

It would also be nice to add support for this to tools/perf/ (so
that 'trace' displays such entries in a, with a
minimum testcase for 'perf test' as well.

Perhaps also add a small sub-utility to inject such events from
the command line, such as:

trace user-event "this is a test message"

('trace' is a shortcut command for 'perf trace'.)

It would have a usecase straight away: perf could be used to
easily trace script execution for example.

For that probably another mode of user event generation would be
needed as well: a process that has no access to any perf fds
should still be able to generate user events, if the
profiling/tracing context has permitted that. In this case we'd
inject the event either into the first, or all currently active
events (but only once per output buffer, or so).


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