powerpc/imc: Add documentation for IMC and trace-mode

Documentation for IMC (In-Memory Collection Counters) infrastructure
and trace-mode of IMC.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
[mpe: Convert to rst, minor rewording, make PMI example more concise]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191028100816.6270-1-mpe@ellerman.id.au

1 files changed, 199 insertions, 0 deletions

diff --git a/Documentation/powerpc/imc.rst b/Documentation/powerpc/imc.rst
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+.. SPDX-License-Identifier: GPL-2.0
+.. _imc:
+
+===================================
+IMC (In-Memory Collection Counters)
+===================================
+
+Anju T Sudhakar, 10 May 2019
+
+.. contents::
+    :depth: 3
+
+
+Basic overview
+==============
+
+IMC (In-Memory collection counters) is a hardware monitoring facility that
+collects large numbers of hardware performance events at Nest level (these are
+on-chip but off-core), Core level and Thread level.
+
+The Nest PMU counters are handled by a Nest IMC microcode which runs in the OCC
+(On-Chip Controller) complex. The microcode collects the counter data and moves
+the nest IMC counter data to memory.
+
+The Core and Thread IMC PMU counters are handled in the core. Core level PMU
+counters give us the IMC counters' data per core and thread level PMU counters
+give us the IMC counters' data per CPU thread.
+
+OPAL obtains the IMC PMU and supported events information from the IMC Catalog
+and passes on to the kernel via the device tree. The event's information
+contains:
+
+- Event name
+- Event Offset
+- Event description
+
+and possibly also:
+
+- Event scale
+- Event unit
+
+Some PMUs may have a common scale and unit values for all their supported
+events. For those cases, the scale and unit properties for those events must be
+inherited from the PMU.
+
+The event offset in the memory is where the counter data gets accumulated.
+
+IMC catalog is available at:
+	https://github.com/open-power/ima-catalog
+
+The kernel discovers the IMC counters information in the device tree at the
+`imc-counters` device node which has a compatible field
+`ibm,opal-in-memory-counters`. From the device tree, the kernel parses the PMUs
+and their event's information and register the PMU and its attributes in the
+kernel.
+
+IMC example usage
+=================
+
+.. code-block:: sh
+
+  # perf list
+  [...]
+  nest_mcs01/PM_MCS01_64B_RD_DISP_PORT01/            [Kernel PMU event]
+  nest_mcs01/PM_MCS01_64B_RD_DISP_PORT23/            [Kernel PMU event]
+  [...]
+  core_imc/CPM_0THRD_NON_IDLE_PCYC/                  [Kernel PMU event]
+  core_imc/CPM_1THRD_NON_IDLE_INST/                  [Kernel PMU event]
+  [...]
+  thread_imc/CPM_0THRD_NON_IDLE_PCYC/                [Kernel PMU event]
+  thread_imc/CPM_1THRD_NON_IDLE_INST/                [Kernel PMU event]
+
+To see per chip data for nest_mcs0/PM_MCS_DOWN_128B_DATA_XFER_MC0/:
+
+.. code-block:: sh
+
+  # ./perf stat -e "nest_mcs01/PM_MCS01_64B_WR_DISP_PORT01/" -a --per-socket
+
+To see non-idle instructions for core 0:
+
+.. code-block:: sh
+
+  # ./perf stat -e "core_imc/CPM_NON_IDLE_INST/" -C 0 -I 1000
+
+To see non-idle instructions for a "make":
+
+.. code-block:: sh
+
+  # ./perf stat -e "thread_imc/CPM_NON_IDLE_PCYC/" make
+
+
+IMC Trace-mode
+===============
+
+POWER9 supports two modes for IMC which are the Accumulation mode and Trace
+mode. In Accumulation mode, event counts are accumulated in system Memory.
+Hypervisor then reads the posted counts periodically or when requested. In IMC
+Trace mode, the 64 bit trace SCOM value is initialized with the event
+information. The CPMCxSEL and CPMC_LOAD in the trace SCOM, specifies the event
+to be monitored and the sampling duration. On each overflow in the CPMCxSEL,
+hardware snapshots the program counter along with event counts and writes into
+memory pointed by LDBAR.
+
+LDBAR is a 64 bit special purpose per thread register, it has bits to indicate
+whether hardware is configured for accumulation or trace mode.
+
+LDBAR Register Layout
+---------------------
+
+  +-------+----------------------+
+  | 0     | Enable/Disable       |
+  +-------+----------------------+
+  | 1     | 0: Accumulation Mode |
+  |       +----------------------+
+  |       | 1: Trace Mode        |
+  +-------+----------------------+
+  | 2:3   | Reserved             |
+  +-------+----------------------+
+  | 4-6   | PB scope             |
+  +-------+----------------------+
+  | 7     | Reserved             |
+  +-------+----------------------+
+  | 8:50  | Counter Address      |
+  +-------+----------------------+
+  | 51:63 | Reserved             |
+  +-------+----------------------+
+
+TRACE_IMC_SCOM bit representation
+---------------------------------
+
+  +-------+------------+
+  | 0:1   | SAMPSEL    |
+  +-------+------------+
+  | 2:33  | CPMC_LOAD  |
+  +-------+------------+
+  | 34:40 | CPMC1SEL   |
+  +-------+------------+
+  | 41:47 | CPMC2SEL   |
+  +-------+------------+
+  | 48:50 | BUFFERSIZE |
+  +-------+------------+
+  | 51:63 | RESERVED   |
+  +-------+------------+
+
+CPMC_LOAD contains the sampling duration. SAMPSEL and CPMCxSEL determines the
+event to count. BUFFERSIZE indicates the memory range. On each overflow,
+hardware snapshots the program counter along with event counts and updates the
+memory and reloads the CMPC_LOAD value for the next sampling duration. IMC
+hardware does not support exceptions, so it quietly wraps around if memory
+buffer reaches the end.
+
+*Currently the event monitored for trace-mode is fixed as cycle.*
+
+Trace IMC example usage
+=======================
+
+.. code-block:: sh
+
+  # perf list
+  [....]
+  trace_imc/trace_cycles/                            [Kernel PMU event]
+
+To record an application/process with trace-imc event:
+
+.. code-block:: sh
+
+  # perf record -e trace_imc/trace_cycles/ yes > /dev/null
+  [ perf record: Woken up 1 times to write data ]
+  [ perf record: Captured and wrote 0.012 MB perf.data (21 samples) ]
+
+The `perf.data` generated, can be read using perf report.
+
+Benefits of using IMC trace-mode
+================================
+
+PMI (Performance Monitoring Interrupts) interrupt handling is avoided, since IMC
+trace mode snapshots the program counter and updates to the memory. And this
+also provide a way for the operating system to do instruction sampling in real
+time without PMI processing overhead.
+
+Performance data using `perf top` with and without trace-imc event.
+
+PMI interrupts count when `perf top` command is executed without trace-imc event.
+
+.. code-block:: sh
+
+  # grep PMI /proc/interrupts
+  PMI:          0          0          0          0   Performance monitoring interrupts
+  # ./perf top
+  ...
+  # grep PMI /proc/interrupts
+  PMI:      39735       8710      17338      17801   Performance monitoring interrupts
+  # ./perf top -e trace_imc/trace_cycles/
+  ...
+  # grep PMI /proc/interrupts
+  PMI:      39735       8710      17338      17801   Performance monitoring interrupts
+
+
+That is, the PMI interrupt counts do not increment when using the `trace_imc` event.