[PATCH V2 7/7] powerpc/perf/vpa-dtl: Add documentation for VPA dispatch trace log PMU

[Athira Rajeev <atrajeev@linux.ibm.com>]

Documentation for vpa-dtl (Virtual Processor Area - Dispatch Trace Log)
PMU interface. And how it can be used to collect the distrace trace log
entries in perf data, how to process/report as part of perf report/perf
script.

Signed-off-by: Athira Rajeev <atrajeev@linux.ibm.com>
---
 Documentation/arch/powerpc/index.rst   |   1 +
 Documentation/arch/powerpc/vpa-dtl.rst | 155 +++++++++++++++++++++++++
 2 files changed, 156 insertions(+)
 create mode 100644 Documentation/arch/powerpc/vpa-dtl.rst

diff --git a/Documentation/arch/powerpc/index.rst b/Documentation/arch/powerpc/index.rst
index 53fc9f89f3e4..1be2ee3f0361 100644
--- a/Documentation/arch/powerpc/index.rst
+++ b/Documentation/arch/powerpc/index.rst
@@ -37,6 +37,7 @@ powerpc
     vas-api
     vcpudispatch_stats
     vmemmap_dedup
+    vpa-dtl
 
     features
 
diff --git a/Documentation/arch/powerpc/vpa-dtl.rst b/Documentation/arch/powerpc/vpa-dtl.rst
new file mode 100644
index 000000000000..98a8550ae1cc
--- /dev/null
+++ b/Documentation/arch/powerpc/vpa-dtl.rst
@@ -0,0 +1,155 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. _htm:
+
+===================================
+DTL (Dispatch Trace Log)
+===================================
+
+Athira Rajeev, 19 April 2025
+
+.. contents::
+    :depth: 3
+
+
+Basic overview
+==============
+
+The pseries Shared Processor Logical Partition(SPLPAR) machines can
+retrieve a log of dispatch and preempt events from the hypervisor
+using data from Disptach Trace Log(DTL) buffer. With this information,
+user can retrieve when and why each dispatch & preempt has occurred.
+The vpa-dtl PMU exposes the Virtual Processor Area(VPA) DTL counters
+via perf.
+
+Infrastructure used
+===================
+
+The VPA DTL PMU counters do not interrupt on overflow or generate any
+PMI interrupts. Therefore, hrtimer is used to poll the DTL data. The timer
+nterval can be provided by user via sample_period field in nano seconds.
+vpa dtl pmu has one hrtimer added per vpa-dtl pmu thread. DTL (Dispatch
+Trace Log) contains information about dispatch/preempt, enqueue time etc.
+We directly copy the DTL buffer data as part of auxiliary buffer and it
+will be processed later. This will avoid time taken to create samples
+in the kernel space. The PMU driver collecting Dispatch Trace Log (DTL)
+entries makes use of AUX support in perf infrastructure. On the tools side,
+this data is made available as PERF_RECORD_AUXTRACE records.
+
+To correlate each DTL entry with other events across CPU's, an auxtrace_queue
+is created for each CPU. Each auxtrace queue has a array/list of auxtrace buffers.
+All auxtrace queues is maintained in auxtrace heap. The queues are sorted
+based on timestamp. When the different PERF_RECORD_XX records are processed,
+compare the timestamp of perf record with timestamp of top element in the
+auxtrace heap so that DTL events can be co-related with other events
+Process the auxtrace queue if the timestamp of element from heap is
+lower than timestamp from entry in perf record. Sometimes it could happen that
+one buffer is only partially processed. if the timestamp of occurrence of
+another event is more than currently processed element in the queue, it will
+move on to next perf record. So keep track of position of buffer to continue
+processing next time. Update the timestamp of the auxtrace heap with the timestamp
+of last processed entry from the auxtrace buffer.
+
+This infrastructure ensures dispatch trace log entries can be correlated
+and presented along with other events like sched.
+
+vpa-dtl PMU example usage
+=========================
+
+.. code-block:: sh
+
+  # ls /sys/devices/vpa_dtl/
+  events  format  perf_event_mux_interval_ms  power  subsystem  type  uevent
+
+
+To capture the DTL data using perf record:
+.. code-block:: sh
+
+  # ./perf record -a -e sched:*,vpa_dtl/dtl_all/ -c 1000000000 sleep 1
+
+The result can be interpreted using perf record. Snippet of perf report -D:
+
+.. code-block:: sh
+
+  # ./perf report -D
+
+There are different PERF_RECORD_XX records. In that records corresponding to
+auxtrace buffers includes:
+
+1. PERF_RECORD_AUX
+   Conveys that new data is available in AUX area
+
+2. PERF_RECORD_AUXTRACE_INFO
+   Describes offset and size of auxtrace data in the buffers
+
+3. PERF_RECORD_AUXTRACE
+   This is the record that defines the auxtrace data which here in case of
+   vpa-dtl pmu is dispatch trace log data.
+
+Snippet from perf report -D showing the PERF_RECORD_AUXTRACE dump
+
+.. code-block:: sh
+
+0 0 0x39b10 [0x30]: PERF_RECORD_AUXTRACE size: 0x690  offset: 0  ref: 0  idx: 0  tid: -1  cpu: 0
+.
+. ... VPA DTL PMU data: size 1680 bytes, entries is 35
+.  00000000: boot_tb: 21349649546353231, tb_freq: 512000000
+.  00000030: dispatch_reason:decrementer interrupt, preempt_reason:H_CEDE, enqueue_to_dispatch_time:7064, ready_to_enqueue_time:187, waiting_to_ready_time:6611773
+.  00000060: dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:146, ready_to_enqueue_time:0, waiting_to_ready_time:15359437
+.  00000090: dispatch_reason:decrementer interrupt, preempt_reason:H_CEDE, enqueue_to_dispatch_time:4868, ready_to_enqueue_time:232, waiting_to_ready_time:5100709
+.  000000c0: dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:179, ready_to_enqueue_time:0, waiting_to_ready_time:30714243
+.  000000f0: dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:197, ready_to_enqueue_time:0, waiting_to_ready_time:15350648
+.  00000120: dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:213, ready_to_enqueue_time:0, waiting_to_ready_time:15353446
+.  00000150: dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:212, ready_to_enqueue_time:0, waiting_to_ready_time:15355126
+.  00000180: dispatch_reason:decrementer interrupt, preempt_reason:H_CEDE, enqueue_to_dispatch_time:6368, ready_to_enqueue_time:164, waiting_to_ready_time:5104665
+
+Above is representation of dtl entry of below format:
+
+struct dtl_entry {
+        u8      dispatch_reason;
+        u8      preempt_reason;
+        u16     processor_id;
+        u32     enqueue_to_dispatch_time;
+        u32     ready_to_enqueue_time;
+        u32     waiting_to_ready_time;
+        u64     timebase;
+        u64     fault_addr;
+        u64     srr0;
+        u64     srr1;
+};
+
+First two fields represent the dispatch reason and preempt reason. The post
+processing of PERF_RECORD_AUXTRACE records will translate to meaningful data
+for user to consume.
+
+Visualize the dispatch trace log entries with perf report
+=========================================================
+
+.. code-block:: sh
+
+  # ./perf record -a -e sched:*,vpa_dtl/dtl_all/ -c 1000000000 sleep 1
+  [ perf record: Woken up 1 times to write data ]
+  [ perf record: Captured and wrote 0.300 MB perf.data ]
+
+  # ./perf report
+  # Samples: 321  of event 'vpa-dtl'
+  # Event count (approx.): 321
+  #
+  # Children      Self  Command  Shared Object      Symbol
+  # ........  ........  .......  .................  ..............................
+  #
+     100.00%   100.00%  swapper  [kernel.kallsyms]  [k] plpar_hcall_norets_notrace
+
+Visualize the dispatch trace log entries with perf script
+=========================================================
+
+.. code-block:: sh
+
+   # ./perf script
+     migration/9      67 [009] 105373.359903:                     sched:sched_waking: comm=perf pid=13418 prio=120 target_cpu=009
+     migration/9      67 [009] 105373.359904:               sched:sched_migrate_task: comm=perf pid=13418 prio=120 orig_cpu=9 dest_cpu=10
+     migration/9      67 [009] 105373.359907:               sched:sched_stat_runtime: comm=migration/9 pid=67 runtime=4050 [ns]
+     migration/9      67 [009] 105373.359908:                     sched:sched_switch: prev_comm=migration/9 prev_pid=67 prev_prio=0 prev_state=S ==> next_comm=swapper/9 next_pid=0 next_prio=120
+            :256     256 [016] 105373.359913:                                vpa-dtl: timebase: 21403600706628832 dispatch_reason:decrementer interrupt, preempt_reason:H_CEDE, enqueue_to_dispatch_time:4854,                        ready_to_enqueue_time:139, waiting_to_ready_time:511842115 c0000000000fcd28 plpar_hcall_norets_notrace+0x18 ([kernel.kallsyms])
+            :256     256 [017] 105373.360012:                                vpa-dtl: timebase: 21403600706679454 dispatch_reason:priv doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:236,                         ready_to_enqueue_time:0, waiting_to_ready_time:133864583 c0000000000fcd28 plpar_hcall_norets_notrace+0x18 ([kernel.kallsyms])
+            perf   13418 [010] 105373.360048:               sched:sched_stat_runtime: comm=perf pid=13418 runtime=139748 [ns]
+            perf   13418 [010] 105373.360052:                     sched:sched_waking: comm=migration/10 pid=72 prio=0 target_cpu=010
-- 
2.47.1