[PATCH V3 0/6] perf/tools: Add interface to expose vpa dtl
tejas05
tejas05 at linux.ibm.com
Wed Sep 17 19:01:14 AEST 2025
> 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.
>
> - Patch 1 to 6 is perf tools side code changes to enable perf
> report/script on perf.data file
>
> Kernel and tools patches is separated. Kernel patches are posted here :
> V2: https://lore.kernel.org/linux-perf-users/20250915072224.98958-1-atrajeev@linux.ibm.com/T/#t
> V3: https://lore.kernel.org/linux-perf-users/2D40E056-6194-40CD-BF72-B474A3ACDCAA@linux.ibm.com/T/#t
>
> 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
> interval 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 corelate 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 corelated
> and presented along with other events like sched.
>
> vpa-dtl PMU example usage
>
> # ls /sys/devices/vpa_dtl/
> events format perf_event_mux_interval_ms power subsystem type uevent
>
>
> To capture the DTL data using perf record:
>
> # ./perf record -a -e sched:*,vpa_dtl/dtl_all/ -c 1000000000 sleep 1
>
> The result can be interpreted using perf report. Snippet of perf report -D:
>
> # ./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
>
> 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
> procecssing of PERF_RECORD_AUXTRACE records will translate to meaninful data
> for user to consume.
>
> Visualize the dispatch trace log entries with perf report:
> # ./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:
>
> # ./perf script
> perf 13322 [002] 233.835807: sched:sched_switch: perf:13322 [120] R ==> migration/2:27 [0]
> migration/2 27 [002] 233.835811: sched:sched_migrate_task: comm=perf pid=13322 prio=120 orig_cpu=2 dest_cpu=3
> migration/2 27 [002] 233.835818: sched:sched_stat_runtime: comm=migration/2 pid=27 runtime=9214 [ns]
> migration/2 27 [002] 233.835819: sched:sched_switch: migration/2:27 [0] S ==> swapper/2:0 [120]
> swapper 0 [002] 233.835822: vpa-dtl: timebase: 338954486062657 dispatch_reason:decrementer_interrupt, preempt_reason:H_CEDE, enqueue_to_dispatch_time:435, ready_to_enqueue_time:0, waiting_to_ready_time:34775058, processor_id: 202 c0000000000f8094 plpar_hcall_norets_notrace+0x18 ([kernel.kallsyms])
> swapper 0 [001] 233.835886: vpa-dtl: timebase: 338954486095398 dispatch_reason:priv_doorbell, preempt_reason:H_CEDE, enqueue_to_dispatch_time:542, ready_to_enqueue_time:0, waiting_to_ready_time:1245360, processor_id: 201 c0000000000f8094 plpar_hcall_norets_notrace+0x18 ([kernel.kallsyms])
>
> Changelog is added for V3 in each individual patch
>
> Thanks
> Athira
>
> Athira Rajeev (6):
> tools/perf: Add basic CONFIG_AUXTRACE support for VPA pmu on powerpc
> tools/perf: process auxtrace events and display in perf report -D
> tools/perf: Add event name as vpa-dtl of PERF_TYPE_SYNTH type to
> present DTL samples
> tools/perf: Allocate and setup aux buffer queue to help co-relate with
> other events across CPU's
> tools/perf: Process the DTL entries in queue and deliver samples
> tools/perf: Enable perf script to present the DTL entries
>
> tools/perf/arch/powerpc/util/Build | 1 +
> tools/perf/arch/powerpc/util/auxtrace.c | 103 ++++
> tools/perf/builtin-script.c | 30 +
> tools/perf/util/Build | 1 +
> tools/perf/util/auxtrace.c | 4 +
> tools/perf/util/auxtrace.h | 1 +
> tools/perf/util/event.h | 20 +
> tools/perf/util/powerpc-vpadtl.c | 734 ++++++++++++++++++++++++
> tools/perf/util/powerpc-vpadtl.h | 23 +
> 9 files changed, 917 insertions(+)
> create mode 100644 tools/perf/arch/powerpc/util/auxtrace.c
> create mode 100644 tools/perf/util/powerpc-vpadtl.c
> create mode 100644 tools/perf/util/powerpc-vpadtl.h
Hi Athira,
I have tested the above patchset on the mainline kernel [ 6.17.0-rc2],
it is working fine. Please add the tag below, for the entire series.
Tested-by: Tejas Manhas <tejas05 at linux.ibm.com>
Thanks & Regards,
Tejas
>
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