Re: [PATCH V3 0/7] Add interface to expose vpa dtl counters via

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 15/09/2025 à 12:29, Athira Rajeev a écrit :
> 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.
> 
> - Patches 1 to 6 has powerpc PMU driver code changes to capture DTL
>    trace in perf.data. And patch 7 has documentation update.
> 
> 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.
> 
> With the kernel changes;
> 
>    # ls /sys/devices/vpa_dtl/
>    events  format  perf_event_mux_interval_ms  power  subsystem  type  uevent
> 
> Thanks
> Athira

What is the difference between this version of the series and the V2 
sent 3 hours ago ?

Christophe