157 lines
7.6 KiB
ReStructuredText
157 lines
7.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
.. _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
|
|
|
|
# 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
|