Re: [PATCH v2 7/7] Documentation: coresight: Add PID tracing description

Hi,

On Thu, 4 Feb 2021 at 11:08, Suzuki K Poulose [off-list ref] wrote:

On 2/4/21 4:09 AM, Leo Yan wrote:

quoted

Hi Mike,

On Wed, Feb 03, 2021 at 05:39:54PM +0000, Mike Leach wrote:

[...]

quoted

quoted

+2.2) Tracing PID
+
+When the kernel is running at EL2 with Virtualization Host Extensions (VHE),
+perf records CONTEXTIDR_EL2 in the trace data and can be used as PID when
+decoding; and if the kernel is running at EL1 with nVHE, CONTEXTIDR_EL1 is
+traced for PID.
+

Would this introductory paragraph be better if is explained where the
kernel stores the PID for the different levels, then we logically move
on to how to trace this in perf.

e.g:-

"The lernel can be built to write the PID value into the PE ContextID registers.
For a kernel running at EL1, the PID is stored in CONTEXTIDR_EL1.
A PE may implement ARM Virtualisation Host Extensions (VHE), were the
kernel can run at EL2 as a virtualisation host.
In this case the PID value is stored in CONTEXTIDR_EL2.
perf provides PMU options which program the ETM to insert these values
into the trace data."

Will in next spin; thanks a lot for writing up!

quoted

quoted

+To support tracing PID for the kernel runs at different exception levels,
+the PMU formats are defined as follow:
+
+  "contextid1": Available on both EL1 kernel and EL2 kernel.  When the
+                kernel is running at EL1, "contextid1" enables the PID
+                tracing; when the kernel is running at EL2, this enables
+                tracing the PID of guest applications.
+
+  "contextid2": Only usable when the kernel is running at EL2.  When
+                selected, enables PID tracing on EL2 kernel.
+
+  "contextid":  Will be an alias for the option that enables PID
+                tracing.  I.e,
+                contextid == contextid1, on EL1 kernel.
+                contextid == contextid2, on EL2 kernel.
+
+The perf tool automatically sets corresponding bit for the "contextid" config,
+therefore, the user doesn't have to bother which EL the kernel is running.
+
+  i.e, perf record -e cs_etm/contextid/u -- uname
+    or perf record -e cs_etm//u -- uname
+
+will always do the "PID" tracing, independent of the kernel EL.
+

This is telling me that both cs_etm// and cs_etm/contextid/ have the
same effect - trace PID. Is this correct?

Just to make this clear, this is not a side effect of the patch.

Which is fine - but the documentation should accurately reflect what
is happening on the system.
This is a new paragraph about the PID tracing or otherwise, Even if
some of the effects pre-date this patch, they have to be accurately
communicated.
I am also reading the new paragraph in the context of the rest of the
coresight.rst document - which is a user level document explaining the
basic operation of the coresight system and tools.
This document mentions no other perf command line parameters relevant
to coresight other than the @sink option.It actually calls out to the
OpenCSD docs to provide further information.

The perf
tool driver automatically adds the "contextid" tracing and timestamp for
"system wide" and process bound events, as they traces get mixed into
the single sink. So these options are added implicitly by the perf tool
to make the decoding easier.

That's fine - I have no problem with contextID trace enabled by
default. Context ID is relatively low overhead - and only emitted at
start of trace  / context changes.
But the explanation of the parameters currently reads as though they
always have an effect - and not putting them in there will omit the
effect - unless you spot the very subtle line at the end.

The user does not need to know about parameters that have no effect!

Perhaps a better approach would be to explain the above - an explicit
statement that "perf will always enable PID/ contextID tracing at the
relevant EL - but for EL2 it is possible to make specific adjustments
using parameters......."

Cheers

Mike

quoted

Correct.

quoted

If so, then contextid, contextid1 and contextid2 have no effect except
in specific EL2 circumstances.

These are required when perf tool may not automatically request them.
With this series the EL2 is on par with the EL1, where we get the PID
automatcially in the trace.

And as you rightly said, contextid1, contextid2 are for EL2 specific
usage.

Cheers
Suzuki

quoted

Yes, exactly.

Thanks,
Leo

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quoted

+When the kernel is running at EL2 with VHE, if user wants to trace both the
+PIDs for both host and guest, the two configs "contextid1" and "contextid2"
+can be set at the same time:
+
+  perf record -e cs_etm/contextid1,contextid2/u -- uname
+

Regards

Mike

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  Generating coverage files for Feedback Directed Optimization: AutoFDO
  ---------------------------------------------------------------------
--
2.25.1

--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK

-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK

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