Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting

[Vince Weaver]

On Wed, 26 Aug 2015, Andrew Morton wrote:

> On Wed, 26 Aug 2015 22:05:13 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > Is this whole thing overkill?  As far as I can see, the problem which is
> > > being addressed only occurs in a couple of places (perf, wifi netlink
> > > handling) and could be addressed with some local pr_debug statements.  ie,
> > > 
> > > #define err_str(e, s) ({
> > > 	if (debugging)
> > > 		pr_debug("%s:%d: error %d (%s)", __FILE__, __LINE__, e, s);
> > > 	e;
> > > })
> > > 
> > > (And I suppose that if this is later deemed inadequate, err_str() could
> > > be made more fancy).
> > 
> > Not really. That is something that's limited to root. Whereas the
> > problem is very much wider than that.
> > 
> > If you set one bit wrong in the pretty large perf_event_attr you've got
> > a fair chance of getting -EINVAL on trying to create the event. Good
> > luck finding what you did wrong.
> > 
> > Any user can create events (for their own tasks), this does not require
> > root.
> > 
> > Allowing users to flip your @debugging flag would be an insta DoS.
> > 
> > Furthermore, its very unfriendly in that you have to (manually) go
> > correlate random dmesg output with some program action.
> 
> It depends on who the audience is.  If it's developers who are writing
> userspace perf tooling then all the above won't be an issue.  If it's
> aimed at end users of that tooling then yes.

As a developer of tools that use the perf_event interface directly (PAPI 
and such) I can say this is a common problem (getting unexplained EINVAL 
results) and yes, telling the user to recompile their kernel to enable 
debugging is usually not an option.

I often have to resort to sprinkling the kernel with printks to find the 
source of errors, which is a pain.  It's even more fun when the user's 
setup is slightly different enough that I can't reproduce the issue on a 
local machine, which happens often (due to different kernels, distros 
backporting perf fixes, different hardware, different security settings, 
etc).

Vince
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