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

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Date:	Tue, 23 Sep 2014 18:02:44 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	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

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?

Anyway, I'll post a series showing this solution in a second.

As always, feedback is more than welcome.

Pawel

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