Re: [PATCH v3 0/7] perf: Add ioctl for PMU driver configuration

[Kim Phillips <kim.phillips@arm.com>]

On Mon, 20 Aug 2018 15:36:47 +0100
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:

> On 08/20/2018 03:22 PM, Kim Phillips wrote:
> > On Mon, 20 Aug 2018 11:03:03 +0100
> > Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
> > 
> >> On 08/16/2018 08:28 PM, Mathieu Poirier wrote:
> >>> On Wed, 15 Aug 2018 at 09:28, Kim Phillips <kim.phillips@arm.com> wrote:
> >>>>
> >>>> On Wed, 15 Aug 2018 10:39:13 +0100
> >>>> Will Deacon <will.deacon@arm.com> wrote:
> >>>>
> >>>>> On Tue, Aug 14, 2018 at 01:42:27PM -0600, Mathieu Poirier wrote:
> >>>>>> On Tue, 14 Aug 2018 at 11:09, Kim Phillips <kim.phillips@arm.com> wrote:
> >>>>>>> The other thing that's going on here is that I'm becoming numb to the
> >>>>>>> loathsome "failed to mmap with 12 (Cannot allocate memory)" being
> >>>>>>> returned no matter what the error is/was. E.g., an error that would
> >>>>>>> indicate a sense of non-implementation would be much better
> >>>>>>> appreciated than presumably what the above is doing, i.e., returning
> >>>>>>> -ENOMEM.  That, backed up with specific details in the form of human
> >>>>>>> readable text in dmesg would be *most* welcome.
> >>>>>>
> >>>>>> As part of the refactoring of the code to support CPU-wide scenarios I
> >>>>>> intend to emit better diagnostic messages from the driver.  Modifying
> >>>>>> rb_alloc_aux() to propagate the error message generated by the
> >>>>>> architecture specific PMUs doesn't look hard either and I _may_ get to
> >>>>>> it as part of this work.
> >>>>>
> >>>>> For the record, I will continue to oppose PMU drivers that dump diagnostics
> >>>>> about user-controlled input into dmesg, but the coresight drivers are yours
> >>>>> so it's up to you and I won't get in the way!
> >>>>
> >>>> That sounds technically self-contradicting to me.  Why shouldn't
> >>>> coresight share the same policies as those used for PMU drivers?  Or
> >>>> why not allow the individual vendor PMU driver authors control the
> >>>> level of user-friendliness of their own drivers?
> >>>>
> >>>> That being said, Matheiu, would you accept patches that make coresight
> >>>> more verbose in dmesg?
> >>>
> >>> It depends on the issue you're hoping to address.  I'd rather see the
> >>> root cause of the problem fixed than adding temporary code.  Suzuki
> >>> added the ETR perf API and I'm currently working on CPU-wide
> >>> scenarios.  From there and with regards to what can happen in
> >>> setup_aux(), we should have things covered.
> >>
> >> I think the main issue is the lack of error code propagation from
> >> setup_aux() back to the perf_aux_output_handle_begin(), which always
> >> return -ENOMEM. If we fix that, we could get better idea of whats
> >> wrong.
> > 
> > Why get a better idea when we can get the exact details?
> 
> The different values for error numbers are there for a reason...

But the same error number, e.g., EINVAL, can be returned for different
reasons.

> >> If someone is planning to add verbose messages, they may do so by adding
> >> dev_dbg() / pr_debug(), which can be turned on as and when needed.
> > 
> > I disagree:  that just adds another usage and kernel configuration
> > obstacle.  Why not use pr_err straight up?
> 
> I personally don't agree to usage of pr_err() in paths which are easily
> triggered from user input.

Why not?  pr_* are ratelimited.

> Also, we are moving all the "debugging"
> messages to the dynamic debug, to prevent lockdep splats.

Are you referring to this?:

https://lkml.org/lkml/2018/5/1/73

Re-reading the thread, AFAICT, it was never proven that the splats
occurred due to the dev_infos, and there's no dev_info in this
stacktrace:

https://lkml.org/lkml/2018/5/10/269

But even if it were, wouldn't the splats also occur with dev_dbg?

Kim