Re: [PATCH v3 04/18] perf cs-etm: Use per-CPU queues for per-thread mode

[Leo Yan]

On Mon, Nov 02, 2026 at 12:15:18PM +0000, James Clark wrote:

[...]

> @@ -2215,6 +2209,29 @@ static int cs_etm__get_data_block(struct cs_etm_queue *etmq)
>  	 */
>  	cs_etm__flush_all_stack(etmq);
>  
> +	/*
> +	 * Per-thread mode still uses a queue for each CPU, but that CPU can run
> +	 * different threads. When the TID from the AUX record on a CPU changes,
> +	 * re-initialize the thread using the AUX record/buffer fragment TID so
> +	 * we can start decoding even if the context ID packet was cropped or
> +	 * they're disabled.
> +	 */
> +	if (etm->per_thread_decoding) {
> +		struct cs_etm_traceid_queue *tidq = cs_etm__etmq_get_traceid_queue(etmq,
> +							CS_ETM_PER_THREAD_TRACEID);
> +
> +		if (thread__tid(tidq->decode_thread) != etmq->buffer->tid) {
> +			thread__zput(tidq->frontend_thread);
> +			thread__zput(tidq->decode_thread);
> +			tidq->frontend_thread = machine__findnew_thread(
> +				&etm->session->machines.host, -1,
> +				etmq->buffer->tid);
> +			tidq->decode_thread = machine__findnew_thread(
> +				&etm->session->machines.host, -1,
> +				etmq->buffer->tid);
> +		}
> +	}

Why not use PERF_RECORD_AUX events as an opportunity to update threads,
similar to how we handle PERF_RECORD_SWITCH_CPU_WIDE?

> +
>  	return 1;
>  }
>  
> @@ -3288,7 +3305,9 @@ 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 = cs_etm__get_queue(etm, auxtrace_event->cpu);
> +		/* Old per-thread files have no CPU (before v6.4) */
> +		int queue_idx = sample->cpu == (u32)-1 ? 0 : sample->cpu;
> +		struct cs_etm_queue *etmq = cs_etm__get_queue(etm, queue_idx);

For per-thread mode, auxtrace_event->cpu is -1. So this is why we
resort sample->cpu to retrieve CPU ID. This makes sense to me.