Messages in this thread | | | Date | Wed, 26 Aug 2015 16:50:33 -0400 (EDT) | From | Vince Weaver <> | Subject | Re: [PATCH v2 0/6] perf: Introduce extended syscall error reporting |
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On Wed, 26 Aug 2015, Andrew Morton wrote:
> On Wed, 26 Aug 2015 22:05:13 +0200 Peter Zijlstra <peterz@infradead.org> 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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