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Mon, 15 Sep 2025 10:30:48 GMT Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 82D582004B; Mon, 15 Sep 2025 10:30:48 +0000 (GMT) Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 490A420043; Mon, 15 Sep 2025 10:30:42 +0000 (GMT) Received: from localhost.localdomain (unknown [9.61.244.131]) by smtpav07.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 15 Sep 2025 10:30:41 +0000 (GMT) From: Athira Rajeev To: maddy@linux.ibm.com, linuxppc-dev@lists.ozlabs.org Cc: acme@kernel.org, jolsa@kernel.org, adrian.hunter@intel.com, irogers@google.com, namhyung@kernel.org, linux-perf-users@vger.kernel.org, aboorvad@linux.ibm.com, sshegde@linux.ibm.com, atrajeev@linux.ibm.com, hbathini@linux.vnet.ibm.com, Aditya.Bodkhe1@ibm.com, venkat88@linux.ibm.com, Tejas.Manhas1@ibm.com Subject: [PATCH V3 7/7] powerpc/perf/vpa-dtl: Add documentation for VPA dispatch trace log PMU Date: Mon, 15 Sep 2025 15:59:47 +0530 Message-Id: <20250915102947.26681-8-atrajeev@linux.ibm.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20250915102947.26681-1-atrajeev@linux.ibm.com> References: <20250915102947.26681-1-atrajeev@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=OMsn3TaB c=1 sm=1 tr=0 ts=68c7eadd cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=yJojWOMRYYMA:10 a=VnNF1IyMAAAA:8 a=Qr6oVggghYX5NUaczaUA:9 X-Proofpoint-ORIG-GUID: hIEVi4MyP3rmed9ykC8W-8_lvJchFE93 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTEzMDAwMSBTYWx0ZWRfX4eu1AuNCpVqA cPWIvKu6mEMurD7OZZvw0yqr/eBo5wGR6WErkdkOpdjsL1p0Yaqx/zkA8nAk9BgTZIQMk54iR3y eCaAIXET33FYp+dsdL9SBdrrl8eJaIcfTrf9r3VDirVBOb4StQj2xxwXujQGSYzD4H3bElXn5oF J2GGqjsA26lY2HDPJLbJzLUco/Yb1xS5mPUt5jMzh/YAkbuUAns08IClrXleY54aCXuLV7ipe0g kfG0/rqUL1WdKteaAxvl0tg5mXDNs2CMKyXEJhYC0rAdHRDGGT/Km8xBRd2ttG4pFnHj9KzHIaG VQCM21BYGZItI75e5m40nQXB0ZMy4eaMnKsuUtT2Y/VJr1ntjoFLTMLyoC9GadZBihRTUrrXhwk oP6QyH9A X-Proofpoint-GUID: iTvv51OWbGAYytO6efjDXlU61jn1ROrv X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-15_04,2025-09-12_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 suspectscore=0 spamscore=0 priorityscore=1501 adultscore=0 impostorscore=0 clxscore=1015 malwarescore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2507300000 definitions=main-2509130001 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 --- Changelog: v2 -> v3: Updated to fix warning from "make htmdocs" Documentation/arch/powerpc/index.rst | 1 + Documentation/arch/powerpc/vpa-dtl.rst | 156 +++++++++++++++++++++++++ 2 files changed, 157 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..58d0022f993a --- /dev/null +++ b/Documentation/arch/powerpc/vpa-dtl.rst @@ -0,0 +1,156 @@ +.. 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 -- 2.47.1