linux-kernel - Re: [RFD] perf syscall error handling

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Date:	Thu, 30 Oct 2014 21:16:36 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Matt Fleming <matt@...sole-pimps.org>,
	Andy Lutomirski <luto@...capital.net>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFD] perf syscall error handling

On Thu, 30 Oct 2014, Peter Zijlstra wrote:

> So would something simple, like an offset into the struct
> perf_event_attr pointing at the current field we're trying to process
> make sense? Maybe with negative offsets to indicate the syscall
> arguments?
> 
> That would narrow down the 'WTF is wrong noaw' a lot I think. But then,
> I've not actually done a lot of userspace the last few years, so maybe
> I'm just dreaming things.

well, as someone who spends a lot of time in userspace trying to help 
people who report probems like 'perf_event_open() returns EINVAL, what's 
wrong' I can say pretty much anything will be an improvement.

What would really help is if we could somehow return the 
filename/line-number of whatever source code file that's setting errno.

Even if perf_event_open() told me that hey, we're getting EOPNOTSUPP due 
to the precise_ip parameter (something that happened just yesterday) it's 
still a lot of grepping and poking around source files to find out what's 
going on.  It would be much better if it just told me the issue was at
kernel/events/core.c line 995 or so, but I'm not sure how you could pass 
that back to the user, and one could argue it wouldn't help much the 
average user without a kernel tree lying around.

Vince
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