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

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Date:   Mon, 20 Aug 2018 15:36:47 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Kim Phillips <kim.phillips@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>, ast@...nel.org,
        Greg KH <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, suzuki.poulosi@....com,
        linux-s390@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 0/7] perf: Add ioctl for PMU driver configuration

On 08/20/2018 03:22 PM, Kim Phillips wrote:
> On Mon, 20 Aug 2018 11:03:03 +0100
> Suzuki K Poulose <suzuki.poulose@....com> wrote:
> 
>> On 08/16/2018 08:28 PM, Mathieu Poirier wrote:
>>> On Wed, 15 Aug 2018 at 09:28, Kim Phillips <kim.phillips@....com> wrote:
>>>>
>>>> On Wed, 15 Aug 2018 10:39:13 +0100
>>>> Will Deacon <will.deacon@....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@....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...

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

Suzuki