linux-kernel - Re: [PATCH] perf: Introduce function to report unsupported syscall attribute flags

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Date:	Wed, 12 Sep 2012 15:02:53 +0200
From:	Robert Richter <robert.richter@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] perf: Introduce function to report unsupported syscall
 attribute flags

On 12.09.12 13:20:43, Peter Zijlstra wrote:
> On Wed, 2012-09-12 at 13:01 +0200, Robert Richter wrote:
> > +       if (notsup)
> > +               pr_warn("perf: unsupported attribute flags: %016llx\n", notsup); 
> 
> This is a dmesg DoS..

This could be avoided by introducing a cpuinfo like sysfs file for
each pmu:

 /sys/bus/event_source/devices/*/flags

Then, userspace should know how to correctly setup the syscall. All
supported attributes would be known to perf and messages would thrown
only if something goes unexpected wrong.

I suggested this already here:

 https://lkml.org/lkml/2012/8/3/214
 https://lkml.org/lkml/2012/9/6/472

> I'm also not sure dmesg is the right way.. could we not somehow change
> the attrs to provide better diagnostic?

I found this discussion without a solution for the problem:

 http://lwn.net/Articles/374794/

Other options could be pr_debug() or trace_printk()'s for debugging
purposes. Or an error report in sysfs:

 cat /sys/bus/event_source/devices/*/log

But the pmu type needs to be known for this.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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