Re: [PATCH 1/2] perf: return ENOENT instead of ENOTSUPP

[Hendrik Brueckner <brueckner@linux.vnet.ibm.com>]

On Thu, Jun 11, 2015 at 12:25:01PM +0200, Peter Zijlstra wrote:
> On Thu, 2015-06-11 at 11:59 +0200, Hendrik Brueckner wrote:
> > The ENOTSUPP (which actually should be EOPNOTSUPP for user space) does not
> > trigger a fallback event selection, for example, by perf record.
> > If hardware support for the cycles perf event is available, but the hardware
> > does not provide interrupts, returning ENOTSUPP causes perf to end.  Returning
> > ENOENT causes the perf tool to fallback to a software-based cycle PMU that
> > supports interrupts.
> > 
> > The commit 53b25335dd ("perf: Disable sampled events if no PMU interrupt")
> > introduced that incompatible change.
> 
> That's 3.16

Correct... I recently encountered the problem.

> 
> >  		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
> > -			err = -ENOTSUPP;
> > +			err = -ENOENT;
> >  			goto err_alloc;
> >  		}
> >  	}
> 
> And now you would be changing an API that's been around for at least 4
> releases.

Well... the behavior before 53b25335dd was differently in this regard.  Of
course, the API changed 4 releases ago.   The question here is rather was
this desired or not.  In my mind I considered this problem as a regression.

> 
> Also, I really think -ENOENT is the wrong return here, you're asking for
> things that's not supported, not for something that's not there.

So looks like -ENOTSUPP is the desired API now.  So the problem I'd like
to solve is that there are two different hardware PMUs that support the
"cycles" event.  Just one of them supports sampling of cycles, the other not.

In the past (prior to 3.16), the perf tool tried several PMUs if -ENOENT
was returned.  With 3.16, -ENOTSUPP is returned (which actually should be
-EOPNOTSUPP but different story) and the perf tool exits.

So the question is: what is the desired behavior?

A solution towards the "fallback-behavior" would be to change
perf_init_event() and consider the sampling/non-sampling criteria (in general
pmu->capabilities) when looking for a matching PMU to serve the event?

Thanks and kind regards,
  Hendrik

-- 
Hendrik Brueckner
brueckner@linux.vnet.ibm.com      | IBM Deutschland Research & Development GmbH
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