Re: [PATCH 00/10] coresight: Add new API to allocate trace source ID values

From: Suzuki Kuruppassery Poulose
Date: Tue Mar 22 2022 - 06:43:50 EST


+ Cc: James Clark

Hi Mike,

On 08/03/2022 20:49, Mike Leach wrote:
The current method for allocating trace source ID values to sources is
to use a fixed algorithm for CPU based sources of (cpu_num * 2 + 0x10).
The STM is allocated ID 0x1.

This fixed algorithm is used in both the CoreSight driver code, and by
perf when writing the trace metadata in the AUXTRACE_INFO record.

The method needs replacing as currently:-
1. It is inefficient in using available IDs.
2. Does not scale to larger systems with many cores and the algorithm
has no limits so will generate invalid trace IDs for cpu number > 44.

Thanks for addressing this issue.


Additionally requirements to allocate additional system IDs on some
systems have been seen.

This patch set introduces an API that allows the allocation of trace IDs
in a dynamic manner.

Architecturally reserved IDs are never allocated, and the system is
limited to allocating only valid IDs.

Each of the current trace sources ETM3.x, ETM4.x and STM is updated to use
the new API.

perf handling is changed so that the ID associated with the CPU is read
from sysfs. The ID allocator is notified when perf events start and stop
so CPU based IDs are kept constant throughout any perf session.

For the ETMx.x devices IDs are allocated on certain events
a) When using sysfs, an ID will be allocated on hardware enable, and freed
when the sysfs reset is written.
b) When using perf, ID is allocated on hardware enable, and freed on
hardware disable.

For both cases the ID is allocated when sysfs is read to get the current
trace ID. This ensures that consistent decode metadata can be extracted
from the system where this read occurs before device enable.



Note: This patchset breaks backward compatibility for perf record.
Because the method for generating the AUXTRACE_INFO meta data has
changed, using an older perf record will result in metadata that
does not match the trace IDs used in the recorded trace data.
This mismatch will cause subsequent decode to fail. Older versions of
perf will still be able to decode data generated by the updated system.

I have some concerns over this and the future plans for the dynamic
allocation per sink. i.e., we are breaking/modifying the perf now to
accommodate the dynamic nature of the trace id of a given CPU/ETM.
The proposed approach of exposing this via sysfs may (am not sure if
this would be the case) break for the trace-id per sink change, as a
sink could assign different trace-id for a CPU depending.

So, instead if we make the trace-id available in the perf (say, an new
record format, PERF_RECORD_CS_ETM_TRACEID ?) record, we can rely on the
new packet for the trace-id, irrespective of how that is allocated and
remove the locking/linking of the trace-id with that of the sysfs. This
is not something that exists today. (Ideally it would have been nice to
have some additional fields in RECORD_AUXINFO, but we don't. Instead of
breaking/extending that, we could add a new RECORD).

I believe the packet may need to be generated only once for a session
and that will also allow the flexibility of moving trace-id allocation
around (to a sink in the future).

Thoughts ?

Kind regards
Suzuki




Applies to coresight/next [b54f53bc11a5]
Tested on DB410c

Mike Leach (10):
coresight: trace-id: Add API to dynamically assign trace ID values
coresight: trace-id: Set up source trace ID map for system
coresight: stm: Update STM driver to use Trace ID api
coresight: etm4x: Use trace ID API to dynamically allocate trace ID
coresight: etm3x: Use trace ID API to allocate IDs
coresight: perf: traceid: Add perf notifiers for trace ID
perf: cs-etm: Update event to read trace ID from sysfs
coresight: Remove legacy Trace ID allocation mechanism
coresight: etmX.X: stm: Remove unused legacy source trace ID ops
coresight: trace-id: Add debug & test macros to trace id allocation

drivers/hwtracing/coresight/Makefile | 2 +-
drivers/hwtracing/coresight/coresight-core.c | 64 ++---
.../hwtracing/coresight/coresight-etm-perf.c | 16 +-
drivers/hwtracing/coresight/coresight-etm.h | 3 +-
.../coresight/coresight-etm3x-core.c | 93 ++++---
.../coresight/coresight-etm3x-sysfs.c | 28 +-
.../coresight/coresight-etm4x-core.c | 63 ++++-
.../coresight/coresight-etm4x-sysfs.c | 32 ++-
drivers/hwtracing/coresight/coresight-etm4x.h | 3 +
drivers/hwtracing/coresight/coresight-priv.h | 1 +
drivers/hwtracing/coresight/coresight-stm.c | 49 +---
.../hwtracing/coresight/coresight-trace-id.c | 255 ++++++++++++++++++
.../hwtracing/coresight/coresight-trace-id.h | 69 +++++
include/linux/coresight-pmu.h | 12 -
include/linux/coresight.h | 3 -
tools/perf/arch/arm/util/cs-etm.c | 12 +-
16 files changed, 530 insertions(+), 175 deletions(-)
create mode 100644 drivers/hwtracing/coresight/coresight-trace-id.c
create mode 100644 drivers/hwtracing/coresight/coresight-trace-id.h