vpa-dtl

.. _vpa-dtl:

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

To capture the DTL data using perf record:

.. code-block:: sh

The result can be interpreted using perf record. Snippet of perf report -D

.. code-block:: sh

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

Visualize the dispatch trace log entries with perf script

.. code-block:: sh