[PATCH 04/25] coresight: perf: End AUX output when an event is throttled

[James Clark]

On 15/09/2026 16:48, Leo Yan wrote:
> Since commit 9734e25fbf5a ("perf: Fix the throttle logic for a
> group"), the perf core stops every event in a throttled group by calling
> the PMU stop callback with flags set to zero:
> 
>    perf_event_throttle()
>      `> event->pmu->stop(event, 0)
> 
> The ETM perf stop callback only ends the AUX output handle when
> PERF_EF_UPDATE was set. For throttling with mode is zero, it skips to
> end the AUX handle. When the event is later unthrottled and ETM tracing
> is started again, perf_aux_output_begin() sees the perf handle is nest
> enabled and triggers a warning:
> 
>    [  181.161897] ------------[ cut here ]------------
>    [  181.161906] WARNING: kernel/events/ring_buffer.c:415 at perf_aux_output_begin+0x1dc/0x1f0, CPU#0:2
>    [  181.234120] arm-scmi arm-scmi.1.auto: timed out in resp(caller: do_xfer+0x1a0/0x558)
>    [  181.234223] cpufreq: __target_index: Failed to change cpu frequency: -110
>    [  181.286675] Modules linked in: panfrost tda9950 hdlcd tda998x drm_shmem_helper drm_client_lib cect
>    [  181.314139] CPU: 0 UID: 0 PID: 352 Comm: perf Not tainted 7.1.0-rc1-00058-gdca922e019dd-dirty #11
>    [  181.323915] Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno Development Platform, B5
>    [  181.334732] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>    [  181.341721] pc : perf_aux_output_begin+0x1dc/0x1f0
>    [  181.346547] lr : perf_aux_output_begin+0x9c/0x1f0
>    [  181.351280] sp : ffff800080003be0
>    [  181.354606] x29: ffff800080003be0 x28: ffff0008023fc000 x27: ffff4b45d287f000
>    [  181.361789] x26: ffff0009764787a8 x25: 0000000000000000 x24: ffff00080dc99710
>    [  181.368970] x23: ffffb4c334553000 x22: 0000000000000001 x21: ffff00080dc99710
>    [  181.376151] x20: ffff0009764787a8 x19: ffff000801cc6000 x18: 00000000ffffffff
>    [  181.383332] x17: 0000000000000000 x16: ffffb4c3a118c998 x15: 0000000000000000
>    [  181.390512] x14: 00003d0900000000 x13: 00000001ffff006e x12: 696765625f747570
>    [  181.397692] x11: ffffb4c3a52758d8 x10: 0000000000000108 x9 : ffffb4c3a117f620
>    [  181.404873] x8 : ffff800080003a58 x7 : ffff000808de4000 x6 : 00000000000fffff
>    [  181.412053] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
>    [  181.419232] x2 : 0000000000000007 x1 : 0000000000000000 x0 : 0000000000000007
>    [  181.426411] Call trace:
>    [  181.428868]  perf_aux_output_begin+0x1dc/0x1f0 (P)
>    [  181.433692]  etm_event_start+0xdc/0x290 [coresight]
>    [  181.438665]  perf_event_unthrottle+0x70/0xa8
>    [  181.442964]  perf_event_unthrottle_group+0x40/0x170
>    [  181.447871]  perf_adjust_freq_unthr_events+0x178/0x1a8
>    [  181.453038]  perf_adjust_freq_unthr_context+0x74/0xf8
>    [  181.458120]  perf_event_task_tick+0xa8/0x290
>    [  181.462418]  sched_tick+0x144/0x2c8
>    [  181.465937]  update_process_times+0xc0/0x198
> 
> Reproduce this with the command:
> 
>    perf record -a -e cs_etm/aux-action=start-paused/k \
> 	-e cycles/aux-action=pause,period=3000/ \
> 	-e cycles/aux-action=resume,period=1500/ -- sleep 10
> 
> An update of the sink buffer is unnecessary when PERF_EF_UPDATE is not
> set, but the AUX output operation must still be terminated. End it with
> a size of zero so that a later restart can acquire a new handle.
> 
> Fixes: 9734e25fbf5a ("perf: Fix the throttle logic for a group")
> Signed-off-by: Leo Yan <leo.yan at arm.com>
> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index 09b21a711a8764ea429d712890265c84648e889e..a45f69f39f175f3044322b25fe5413fb97154496 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -748,13 +748,16 @@ static void etm_event_stop(struct perf_event *event, int mode)
>   	 * handle due to lack of buffer space), we don't
>   	 * have to do anything here.
>   	 */
> -	if (handle->event && (mode & PERF_EF_UPDATE)) {
> +	if (!handle->event)
> +		goto out;
> +
> +	if (mode & PERF_EF_UPDATE) {
>   		if (WARN_ON_ONCE(handle->event != event))
> -			return;
> +			goto out;
>   
>   		/* update trace information */
>   		if (!sink_ops(sink)->update_buffer)
> -			return;
> +			goto out;
>   
>   		size = sink_ops(sink)->update_buffer(sink, handle,
>   					      event_data->snk_config);
> @@ -773,8 +776,11 @@ static void etm_event_stop(struct perf_event *event, int mode)
>   			perf_aux_output_end(handle, size);
>   		else
>   			WARN_ON(size);
> +	} else {
> +		perf_aux_output_end(handle, 0);
>   	}
>   
> +out:
>   	/* Disabling the path make its elements available to other sessions */
>   	coresight_disable_path(path);
>   }
> 

Reviewed-by: James Clark <james.clark at linaro.org>