> On 15 Sep 2025, at 12:57 PM, Athira Rajeev <atraj...@linux.ibm.com> wrote:
>
> 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 :
> https://lore.kernel.org/linux-perf-users/20250915072224.98958-1-atraj...@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
> 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 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])
>
> 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 | 114 ++++
> tools/perf/builtin-script.c | 27 +
> 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 | 732 ++++++++++++++++++++++++
> tools/perf/util/powerpc-vpadtl.h | 25 +
> 9 files changed, 925 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
>
> --
> 2.47.1
>
Hello Athira,
Tested this patch set by applying on top of today’s mainline kernel, which was
built by applying dtl enablement in PMU driver for PowerPC. Please refer below
link for the patchiest
https://lore.kernel.org/all/20250915102947.26681-1-atraj...@linux.ibm.com/
And this patchiest works as expected.
As a side note, if for some reason, you are resending this patch set, please
correct the interval spelling in the cover page.
Please add below tag.
Tested-by: Venkat Rao Bagalkote <venka...@linux.ibm.com>
Regards,
Venkat.
