Date: Wed, 26 Aug 2015 16:50:33 -0400 (EDT)
From: Vince Weaver <vince@deater.net>
To: Andrew Morton <akpm@linux-foundation.org>
cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Johannes Berg <johannes@sipsolutions.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, adrian.hunter@intel.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        linux-kernel@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v2 0/6] perf: Introduce extended syscall error
 reporting
In-Reply-To: <20150826132249.fbb2aa602525c7df4a1f86ed@linux-foundation.org>
Message-ID: <alpine.DEB.2.10.1508261646090.1976@pianoman.cluster.toy>
References: <20150825091740.GA23488@gmail.com> <1440495246.2192.13.camel@sipsolutions.net> <20150825100728.GA1820@gmail.com> <1440497981.2192.39.camel@sipsolutions.net> <20150826044948.GC14584@gmail.com> <CA+55aFw--OFczoY=v17+e2-Q3O0GXnMKRuwzpYpB2qKBpZo=fw@mail.gmail.com>
 <1440572775.1932.1.camel@sipsolutions.net> <20150826072020.GA19081@gmail.com> <20150826072656.GA19305@gmail.com> <20150826114111.01675d8eadda78d82933d8a5@linux-foundation.org> <20150826200513.GX16853@twins.programming.kicks-ass.net>
 <20150826132249.fbb2aa602525c7df4a1f86ed@linux-foundation.org>
User-Agent: Alpine 2.10 (DEB 1266 2009-07-14)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2310
Lines: 54

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/