From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH 2/2] perf: Userspace software event and ioctl
Date: Wed, 24 Sep 2014 09:49:42 +0200
Message-ID: <20140924074942.GB3797@gmail.com>
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Cc: Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Richard Cochran <richardcochran-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>, Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Paul Mackerras <paulus-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>, John Stultz <john.stultz-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>, "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, "linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
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* Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org> wrote:

> On Thu, 2014-09-18 at 15:34 +0100, Pawel Moll wrote:
> > This patch adds a PERF_COUNT_SW_USERSPACE_EVENT type,
> > which can be generated by user with PERF_EVENT_IOC_ENTRY
> > ioctl command, which injects an event of said type into
> > the perf buffer.
> 
> It occurred to me last night that currently perf doesn't handle "write"
> syscall at all, while this seems like the most natural way of
> "injecting" userspace events into perf buffer.
> 
> An ioctl would still be needed to set a type of the following events,
> something like:
> 
> 	ioctl(SET_TYPE, 0x42);
> 	write(perf_fd, binaryblob, size);
> 	ioctl(SET_TYPE, 0);
> 	dprintf(perf_fd, "String");
> 
> which is fine for use cases when the type doesn't change often, 
> but would double the amount of syscalls when every single event 
> is of a different type. Perhaps there still should be a 
> "generating ioctl" taking both type and data/size in one go?

Absolutely, there should be a single syscall.

I'd even argue it should be a new prctl(): that way we could both 
generate user events for specific perf fds, but also into any 
currently active context (that allows just generation/injection 
of user events). In the latter case we might have no fd to work 
off from.

And that is actually the really exciting usecase of your patches: 
we could generate user events via simple commands, and any 
external profiler/trace would be able to see them.

Thanks,

	Ingo