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Subject[PATCH v4 0/5] powerpc/perf: IMC trace-mode support
Date
IMC (In-Memory collection counters) is a hardware monitoring facility      
that collects large number of hardware performance events.
POWER9 support 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 CPMC*SEL and CPMC_LOAD in the trace scom, specifies
the event to be monitored and the sampling duration. On each overflow in the
CPMC*SEL, hardware snapshots the program counter along with event counts
and writes into memory pointed by LDBAR. LDBAR has bits to indicate whether
hardware is configured for accumulation or trace mode.
Currently the event monitored for trace-mode is fixed as cycle.

Trace-IMC Implementation:
--------------------------
To enable trace-imc, we need to

* Add trace node in the DTS file for power9, so that the new trace node can
be discovered by the kernel.

Information included in the DTS file are as follows, (a snippet from
the ima-catalog)

TRACE_IMC: trace-events {
#address-cells = <0x1>;
#size-cells = <0x1>;
event at 10200000 {
event-name = "cycles" ;
reg = <0x10200000 0x8>;
desc = "Reference cycles" ;
};
};
trace@0 {
compatible = "ibm,imc-counters";
events-prefix = "trace_";
reg = <0x0 0x8>;
events = < &TRACE_IMC >;
type = <0x2>;
size = <0x40000>;
};

OP-BUILD changes needed to include the "trace node" is already pulled in
to the ima-catalog repo.

ps://github.com/open-power/op-build/commit/d3e75dc26d1283d7d5eb444bff1ec9e40d5dfc07

* Enchance the opal_imc_counters_* calls to support this new trace mode
in imc. Add support to initialize the trace-mode scom.

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 CPMC*SEL determines
the event to count. BUFFRSIZE indicates the memory range. On each overflow,
hardware snapshots program counter along with event counts and update 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.

OPAL support for IMC trace mode is already upstream.

* Set LDBAR spr to enable imc-trace mode.

LDBAR 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

----------------------

PMI interrupt handling is avoided, since IMC trace mode snapshots the
program counter and update to the memory. And this also provide a way for
the operating system to do instruction sampling in real time without
PMI(Performance Monitoring Interrupts) 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.

# cat /proc/interrupts (a snippet from the output)
9944 1072 804 804 1644 804 1306
804 804 804 804 804 804 804
804 804 1961 1602 804 804 1258
[-----------------------------------------------------------------]
803 803 803 803 803 803 803
803 803 803 803 804 804 804
804 804 804 804 804 804 803
803 803 803 803 803 1306 803
803 Performance monitoring interrupts


`perf top` with trace-imc (executed right after 'perf top' without trace-imc event):

# perf top -e trace_imc/trace_cycles/
12.50% [kernel] [k] arch_cpu_idle
11.81% [kernel] [k] __next_timer_interrupt
11.22% [kernel] [k] rcu_idle_enter
10.25% [kernel] [k] find_next_bit
7.91% [kernel] [k] do_idle
7.69% [kernel] [k] rcu_dynticks_eqs_exit
5.20% [kernel] [k] tick_nohz_idle_stop_tick
[-----------------------]

# cat /proc/interrupts (a snippet from the output)

9944 1072 804 804 1644 804 1306
804 804 804 804 804 804 804
804 804 1961 1602 804 804 1258
[-----------------------------------------------------------------]
803 803 803 803 803 803 803
803 803 803 804 804 804 804
804 804 804 804 804 804 803
803 803 803 803 803 1306 803
803 Performance monitoring interrupts

The PMI interrupts count remains the same.


Changelog:
----------
From v3 -> v4:

* trace_imc_refc is introduced. So that even if, core-imc
is disabled, trace-imc can be used.

* trace_imc_pmu_sched_task is removed and opal start/stop
is invoked in trace_imc_event_add/del function.


Suggestions/comments are welcome.

Anju T Sudhakar (4):
powerpc/include: Add data structures and macros for IMC trace mode
powerpc/perf: Rearrange setting of ldbar for thread-imc
powerpc/perf: Trace imc events detection and cpuhotplug
powerpc/perf: Trace imc PMU functions

Madhavan Srinivasan (1):
powerpc/perf: Add privileged access check for thread_imc

arch/powerpc/include/asm/imc-pmu.h | 39 +++
arch/powerpc/include/asm/opal-api.h | 1 +
arch/powerpc/perf/imc-pmu.c | 340 +++++++++++++++++++++-
arch/powerpc/platforms/powernv/opal-imc.c | 3 +
include/linux/cpuhotplug.h | 1 +
5 files changed, 372 insertions(+), 12 deletions(-)
--
2.17.2

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