From: Suzuki K Poulose <firstname.lastname@example.org>
Cc: email@example.com, firstname.lastname@example.org,
Al.Grant@arm.com, email@example.com, firstname.lastname@example.org,
James.Clark@arm.com, email@example.com, firstname.lastname@example.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: <email@example.com> (raw)
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 <firstname.lastname@example.org> 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 <email@example.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:
>>>>> 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
>> 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
>>> 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
>> 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 :
> Kind regards
next prev parent reply other threads:[~2021-07-28 9:24 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-23 12:46 [PATCH v2 00/10] coresight: TRBE and Self-Hosted trace fixes Suzuki K Poulose
2021-07-23 12:46 ` [PATCH v2 01/10] coresight: etm4x: Save restore TRFCR_EL1 Suzuki K Poulose
2021-07-30 3:05 ` Anshuman Khandual
2021-07-23 12:46 ` [PATCH v2 02/10] coresight: etm4x: Use Trace Filtering controls dynamically Suzuki K Poulose
2021-07-30 3:48 ` Anshuman Khandual
2021-07-30 11:29 ` Suzuki K Poulose
2021-07-23 12:46 ` [PATCH v2 03/10] coresight: etm-pmu: Ensure the AUX handle is valid Suzuki K Poulose
2021-07-30 4:14 ` Anshuman Khandual
2021-07-23 12:46 ` [PATCH v2 04/10] coresight: trbe: Ensure the format flag is set on truncation Suzuki K Poulose
2021-07-30 4:26 ` Anshuman Khandual
2021-07-30 11:37 ` Suzuki K Poulose
2021-07-23 12:46 ` [PATCH v2 05/10] coresight: trbe: Drop duplicate TRUNCATE flags Suzuki K Poulose
2021-07-30 4:47 ` Anshuman Khandual
2021-07-30 12:58 ` Suzuki K Poulose
2021-07-23 12:46 ` [PATCH v2 06/10] coresight: trbe: Fix handling of spurious interrupts Suzuki K Poulose
2021-07-30 5:15 ` Anshuman Khandual
2021-07-30 12:57 ` Suzuki K Poulose
2021-07-23 12:46 ` [PATCH v2 07/10] coresight: trbe: Do not truncate buffer on IRQ Suzuki K Poulose
2021-07-26 12:34 ` Mike Leach
2021-07-26 16:01 ` Suzuki K Poulose
2021-07-27 10:46 ` Mike Leach
2021-07-27 13:06 ` Suzuki K Poulose
2021-07-28 9:25 ` Suzuki K Poulose [this message]
2021-07-23 12:46 ` [PATCH v2 08/10] coresight: trbe: Unify the enabling sequence Suzuki K Poulose
2021-07-30 5:40 ` Anshuman Khandual
2021-07-23 12:46 ` [PATCH v2 09/10] coresight: trbe: End the AUX handle on truncation Suzuki K Poulose
2021-07-30 5:54 ` Anshuman Khandual
2021-07-23 12:46 ` [PATCH v2 10/10] coresight: trbe: Prohibit trace before disabling TRBE Suzuki K Poulose
2021-07-30 6:58 ` Anshuman Khandual
2021-07-23 13:45 ` [PATCH v2 00/10] coresight: TRBE and Self-Hosted trace fixes Suzuki K Poulose
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