[PATCH v2 1/2] perf cs-etm: Fix decoding for sparse CPU maps

[Leo Yan]

On Mon, Jan 19, 2026 at 11:55:53AM +0000, James Clark wrote:

[...]

> > >    0 0 0x200 [0x90]: PERF_RECORD_ID_INDEX nr: 4
> > >    ... id: 771  idx: 0  cpu: 2  tid: -1
> > >    ... id: 772  idx: 1  cpu: 3  tid: -1
> > >    ... id: 773  idx: 0  cpu: 2  tid: -1
> > >    ... id: 774  idx: 1  cpu: 3  tid: -1
> > 
> > Seems to me that this patch works around the issue by using the CPU ID
> > instead, but event->auxtrace.idx is broken.
>
> I don't think it's a workaround, I think it should have been written this
> way in the first place.

Agree.  Sorry for confusion in my rely.

[...]

> > > @@ -3086,7 +3086,7 @@ static int cs_etm__queue_aux_fragment(struct perf_session *session, off_t file_o
> > >   	if (aux_offset >= auxtrace_event->offset &&
> > >   	    aux_offset + aux_size <= auxtrace_event->offset + auxtrace_event->size) {
> > > -		struct cs_etm_queue *etmq = etm->queues.queue_array[auxtrace_event->idx].priv;
> > > +		struct cs_etm_queue *etmq = cs_etm__get_queue(etm, auxtrace_event->cpu);

During decoding, cs_etm allocates its own AUX queues and needs to
convert the AUX records into a cs_etm-specific format.

For example:

  event_id=405 idx=0 cpu=0 tid=6673
  event_id=406 idx=1 cpu=1 tid=6673
  event_id=407 idx=2 cpu=3 tid=6673
  event_id=408 idx=3 cpu=4 tid=6673
  event_id=409 idx=4 cpu=5 tid=6673
  `            `      `> CPU ID
   `> Event ID  `> Generic buffer IDs

In this case, I deliberately hotplugged off CPU2.  As a result, the
buffer IDs are consecutive from 0 to 4, while the CPU ID for CPU2 is
missing.  When adding AUX records to the cs_etm queues, we need to
convert from the generic buffer index to the corresponding CPU ID.

So the above change makes sense to me.

> > >   		/*
> > >   		 * If this AUX event was inside this buffer somewhere, create a new auxtrace event
> > > @@ -3095,6 +3095,7 @@ static int cs_etm__queue_aux_fragment(struct perf_session *session, off_t file_o
> > >   		auxtrace_fragment.auxtrace = *auxtrace_event;
> > >   		auxtrace_fragment.auxtrace.size = aux_size;
> > >   		auxtrace_fragment.auxtrace.offset = aux_offset;
> > > +		auxtrace_fragment.auxtrace.idx = etmq->queue_nr;

I am still confused about this.  Because above auxtrace will be queued
on cs_etm queues, should we convert from the generic buffer index to the
cs_etm specific one?  E.g.,

  auxtrace_fragment.auxtrace.idx = auxtrace_event->cpu;

BTW, if my understanding above is valid, it is good to go through the
cs_etm.c file for the "idx <-> CPU ID" conversion.

Thanks,
Leo

> > >   		file_offset += aux_offset - auxtrace_event->offset + auxtrace_event->header.size;
> > >   		pr_debug3("CS ETM: Queue buffer size: %#"PRI_lx64" offset: %#"PRI_lx64
> > > 
> > > -- 
> > > 2.34.1
> > > 
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