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diff --git a/Documentation/powerpc/imc.rst b/Documentation/powerpc/imc.rst new file mode 100644 index 000000000..633bcee7d --- /dev/null +++ b/Documentation/powerpc/imc.rst @@ -0,0 +1,199 @@ +.. 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. |