linux-kernel - Re: [PATCH 2/2] perf: Userspace software event and ioctl

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Date:	Wed, 24 Sep 2014 09:49:42 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Pawel Moll <pawel.moll@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] perf: Userspace software event and ioctl


* Pawel Moll <pawel.moll@....com> 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
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