Re: [PATCH v2 07/10] coresight: trbe: Do not truncate buffer on IRQ

From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: mike.leach@linaro.org
Cc: coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, tamas.zsoldos@arm.com,
	Al.Grant@arm.com, leo.yan@linaro.org, mathieu.poirier@linaro.org,
	anshuman.khandual@arm.com, jinlmao@qti.qualcomm.com,
	James.Clark@arm.com, peterz@infradead.org, will@kernel.org
Subject: Re: [PATCH v2 07/10] coresight: trbe: Do not truncate buffer on IRQ
Date: Wed, 28 Jul 2021 10:25:49 +0100	[thread overview]
Message-ID: <29b80c07-932f-4ba3-9ad6-3edf78b20ee4@arm.com> (raw)
In-Reply-To: <278117cd-677d-c1be-b1e3-ea2a8825fcb1@arm.com>

On 07/27/2021 02:06 PM, Suzuki K Poulose wrote:
> On 27/07/2021 11:46, Mike Leach wrote:
>> HI Suzuki,
>>
>> On Mon, 26 Jul 2021 at 17:01, Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>>
>>> Hi Mike,
>>>
>>> On 26/07/2021 13:34, Mike Leach wrote:
>>>> Hi Suzuki,
>>>>
>>>> On Fri, 23 Jul 2021 at 13:46, Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
>>>>>
>>>>> The TRBE driver marks the AUX buffer as TRUNCATED when we get an IRQ
>>>>> on FILL event. This has rather unwanted side-effect of the event
>>>>> being disabled when there may be more space in the ring buffer.
>>>>>
>>>>> So, instead of TRUNCATE we need a different flag to indicate
>>>>> that the trace may have lost a few bytes (i.e from the point of
>>>>> generating the FILL event until the IRQ is consumed). Anyways, the
>>>>> userspace must use the size from RECORD_AUX headers to restrict
>>>>> the "trace" decoding.
>>>>>
>>>>> Using PARTIAL flag causes the perf tool to generate the
>>>>> following warning:
>>>>>
>>>>>     Warning:
>>>>>     AUX data had gaps in it XX times out of YY!
>>>>>
>>>>>     Are you running a KVM guest in the background?
>>>>>
>>>>> which is pointlessly scary for a user. The other remaining options
>>>>> are :
>>>>>     - COLLISION - Use by SPE to indicate samples collided
>>>>>     - Add a new flag - Specifically for CoreSight, doesn't sound
>>>>>       so good, if we can re-use something.
>>>>>
>>>>
>>>> What is the user visible behaviour when using COLLISION?
>>>
>>> If you meant a Warning from the perf tool (similar to TRUNCATE or
>>> PARTIAL), the answer is none. We could add one in the perf tool
>>> if you think this is necessary.
>>>
>>
>> I do - the problem is that we have replaced a visible warning with a
>> silent failure.
>>
>> While we agree that the side effects of TRUNCATE mean it unfeasible as
>> a solution here - at least the PARTIAL message does give some
>> indication.
>> The average perf user is going to rely on the output from the tool -
>> if there is no warning they will assume all is good, but they have
>> possible non-contiguous trace and no indication of such.
>>
>> Since we are using a collision flag  in a particular context - i.e.
>> coresight trace - we have the chance to provide an appropriate message
>> for this context.
>>
>>>> The TRUNCATE warning is at least accurate - even if the KVM thing is
>>>> something of a red herring.
>>>
>>
>> Sorry - I meant PARTIAL here - but the comment stands otherwise.
>>
>>>
>>>> It is easier to explain a "scary" warning, than try to debug someones
>>>> problems if perf is silent or misleading when using the COLLISION
>>>> flag.
>>>
>>> The RECORD_AUX still has this flag. So, if someone really wanted to
>>> know how many times the TRBE fired the IRQ and thus potentially lost a
>>> few bytes of the trace, they could always look at this.
>>>
>>
>> They could - but how would they know that they needed to - what
>> indicators would they have that the trace was not continuous?
>> The point of the perf tool is that it presents an accurate picture to
>> the user, based on the data collected. Most users aren't going to
>> start digging into the intricacies of the perf data file formats and
>> nor should they have to.
>>
>>> Definitely this is not something similar to "TRUNCATED", which we
>>> realized the hard way, nor the PARTIAL. But the perf tool could
>>> report something similar. Please remember that the perf tool always
>>> uses the "size" field from the RECORD_AUX to limit the trace decoding.
>>>
>>> So, I am not sure how this could create new problems.
>>>
>>
>> There is no issue with decode - but if a user is investigating a
>> problem using trace, they need to be aware that some trace might be
>> dropped.
>> That way they can take mitigating action.
> 
> Agreed. This is something that can be done by the perf tool.

fyi, I have posted a patch to do this here :

[0] https://lkml.kernel.org/r/20210728091219.527886-1-suzuki.poulose@arm.com

Suzuki


> 
> Kind regards
> Suzuki