[Skiboot] [PATCH v2 0/3] skiboot: OPAL support for IMC trace-mode
Stewart Smith
stewart at linux.vnet.ibm.com
Mon Feb 25 16:39:17 AEDT 2019
Anju T Sudhakar <anju at linux.vnet.ibm.com> writes:
> 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, event counted is fixed for cycles and on
> each overflow, hardware snapshots the program counter along with other
> details and writes into memory pointed by LDBAR(ring buffer memory,
> hardware wraps around). LDBAR has bits to indicate the IMC trace-mode.
Are there any tests for op-test to test this functionality?
>
> 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.
>
> Informations 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 at 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 other details 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.
> ----------------
>
> Key benefit of imc trace-mode is, each sample record contains the address
> pointer along with other information. So that, we can profile the IP
> without interrupting the application.
>
> Performance data using 'perf top' with and without trace-imc event:
>
> When the application is monitored with trace-imc event, we dont take any
> PMI interrupts.
>
> PMI interrupts count when `perf top` command is executed without trace-imc event.
>
> # perf top
> 12.53% [kernel] [k] arch_cpu_idle
> 11.32% [kernel] [k] rcu_idle_enter
> 10.76% [kernel] [k] __next_timer_interrupt
> 9.49% [kernel] [k] find_next_bit
> 8.06% [kernel] [k] rcu_dynticks_eqs_exit
> 7.82% [kernel] [k] do_idle
> 5.71% [kernel] [k] tick_nohz_idle_stop_tic
> [-----------------------]
> # 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 (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.
Something like the above, documenting the advantage, should probably
also be in the OPAL docs. i.e. this is a way for OS to look at
performance counters of recent events without having the performance
monitoring interrupts fire. e.g. you could leave it on constantly
without an impact to performance (one hopes).
>
> Changelog:
>
> From v1 -> v2
> ---------------
>
> * Added documentation for the new trace-mode support
> * Updated the commit message in the last patch
>
>
> Anju T Sudhakar (5):
> skiboot/doc: Add documentation for trace-mode in imc.rst
> skiboot: Edit documentation for IMC opal call to include trace-imc
> skiboot: Trace IMC Macro definitions
> skiboot: Enable opal calls to init/start/stop IMC Trace mode
> skiboot: Mambo hack to add trace-imc nodes in the device-tree
>
> doc/imc.rst | 9 +++
> doc/opal-api/opal-imc-counters.rst | 28 ++++----
> external/mambo/skiboot.tcl | 21 ++++++
> hw/imc.c | 103 +++++++++++++++++++++++++++++
> include/imc.h | 30 +++++++++
> include/opal-api.h | 1 +
> 6 files changed, 180 insertions(+), 12 deletions(-)
>
> --
> 2.17.1
>
--
Stewart Smith
OPAL Architect, IBM.
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