[PATCH v2 0/5] alpha/perf: add ProfileMe sampling

[Matt Turner <mattst88@gmail.com>]

The 21264 from EV67 on, and the 21364, have a second performance counter
mode, ProfileMe, alongside the Aggregate mode that the driver uses today.
An Aggregate mode sample reports the PC where the overflow interrupt was
taken. On an EV7 that is anywhere from 15 instructions before to 12
after the one that overflowed the counter, which makes perf annotate
output close to useless. In ProfileMe mode the CPU picks one fetched
instruction, follows it down the pipeline, and reports its PC along with
what happened to it: whether it retired, whether it trapped and why,
whether it was a mispredicted branch, stalled before map, or was killed
on a wrong path, and what PCTR1 counted while it was in flight.

Patch 1 fixes a constant passed to the wrong PALcode call, with no
functional change. Patch 2 adds the sysfs format and event attributes
the perf tool needs to name raw events. Patch 3 adds ProfileMe sampling,
selected with precise_ip or a new profileme config bit, so cycles:p and
perf record's default event use it. Patch 4 attaches the status
registers to each sample as PERF_SAMPLE_RAW data, and patch 5 adds an
option to randomize the sampling interval so that it cannot lock onto a
loop.

A separate series for perf-tools-next adds a decoder for the raw data
to perf report -D and perf script -D, and a script that sums the status
by symbol or by instruction, in the style of DCPI.

The two modes cannot run at the same time, because PCTR_CTL[SL0] sets
the mode for both counters. A ProfileMe event is scheduled alone, and
perf multiplexes it with any other hardware events. perf stat does not
ask for precise events, so it keeps using Aggregate mode.

To try it:

  perf record -e cycles:p -- ./workload
  perf annotate

I tested this on an AlphaServer ES47 (EV7). Samples from a
16-instruction loop land on all 16 instructions rather than on 4 PCs.
DTB misses and unaligned traps are attributed to the faulting load. A
branch on random data shows up as mispredicted in 21% of its samples,
and in 0% with sorted data. A 12 minute perf record -a while building
git took 4.45 million samples with no warnings. Sampling every 10000
instructions, which used to wedge the CPU, ran 66 times without a hang.

Signed-off-by: Matt Turner <mattst88@gmail.com>
---
Changes in v2:
- Patch 1: explain why bit 0 clear means all processes, although the
  Architecture Reference Manual says otherwise.
- Patch 3: fix a CPU wedge during high-rate ProfileMe sampling. The
  ProfileMe counters are now written twice, and a stopped counter is
  moved away from overflow, as EV67 HRM PALcode restrictions 32 and 38
  require.
- Patch 3: set the profiled PC in place in the trap frame of the
  instruction's mode instead of in a pt_regs copy on the stack, so
  sampled registers and stack values are those of the right frame.
  A kernel instruction whose interrupt is taken in user mode gets a
  per-CPU frame holding only its PC, so the user frame is not changed.
- Patch 4: drop the reserved field and pack the 64-bit fields of
  struct alpha_profileme_sample, so they are 8-byte aligned in the
  sample record and the record needs no padding. This changes the raw
  data layout.
- Patch 5: compute the jitter with mul_u64_u32_shr() and saturate the
  sum, so that large periods cannot overflow.
- Link to v1: https://patch.msgid.link/20261028-alpha-profileme-v1-0-d44b52bf2500@gmail.com

To: Richard Henderson <richard.henderson@linaro.org>
To: Matt Turner <mattst88@gmail.com>
To: Magnus Lindholm <linmag7@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@redhat.com>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Namhyung Kim <namhyung@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
To: Jiri Olsa <jolsa@kernel.org>
To: Ian Rogers <irogers@google.com>
To: Adrian Hunter <adrian.hunter@intel.com>
To: James Clark <james.clark@linaro.org>
Cc: linux-alpha@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-perf-users@vger.kernel.org

---
Matt Turner (5):
      alpha/perf: pass PERFMON_CMD_LOGGING_ALL to the logging options call
      alpha/perf: add sysfs format and event attributes
      alpha/perf: implement ProfileMe sampling
      alpha/perf: attach ProfileMe status as PERF_SAMPLE_RAW data
      alpha/perf: optionally randomize the ProfileMe sampling interval

 arch/alpha/include/asm/perf_event.h      |   2 +
 arch/alpha/include/asm/wrperfmon.h       |  14 ++
 arch/alpha/include/uapi/asm/perf_event.h |  31 ++++
 arch/alpha/kernel/perf_event.c           | 309 +++++++++++++++++++++++++++++--
 4 files changed, 345 insertions(+), 11 deletions(-)
---
base-commit: 330517c63aa9b48227b6ab45f01003effac77ae4
change-id: 20261024-alpha-profileme-6e98a977cb54

Best regards,
--  
Matt Turner <mattst88@gmail.com>