[PATCH v2 3/3] perf cs-etm: Prevent and warn on underflows during timestamp calculation.

[Leo Yan]

On Thu, May 13, 2021 at 05:35:20PM +0300, James Clark wrote:
> When a zero timestamp is encountered, warn once. This is to make
> hardware or configuration issues visible. Also suggest that the issue
> can be worked around with the --itrace=Z option.
> 
> When an underflow with a non-zero timestamp occurrs, warn every time.

s/occurrs/occurs

> This is an unexpected scenario, and with increasing timestamps, it's
> unlikely that it would occur more than once, therefore it should be
> ok to warn every time.
> 
> Only try to calculate the timestamp by subtracting the instruction
> count if neither of the above cases are true. This makes attempting
> to decode files with zero timestamps in non-timeless mode
> more consistent. Currently it can half work if the timestamp wraps
> around and becomes non-zero, although the behavior is undefined and
> unpredictable.
> 
> Signed-off-by: James Clark <james.clark at arm.com>

All three patches look good to me:

Reviewed-by: Leo Yan <leo.yan at linaro.org>

> ---
>  .../perf/util/cs-etm-decoder/cs-etm-decoder.c | 45 ++++++++++++++-----
>  1 file changed, 34 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> index b01d363b9301..3e1a05bc82cc 100644
> --- a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> +++ b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> @@ -6,6 +6,7 @@
>   * Author: Mathieu Poirier <mathieu.poirier at linaro.org>
>   */
>  
> +#include <asm/bug.h>
>  #include <linux/coresight-pmu.h>
>  #include <linux/err.h>
>  #include <linux/list.h>
> @@ -17,6 +18,7 @@
>  
>  #include "cs-etm.h"
>  #include "cs-etm-decoder.h"
> +#include "debug.h"
>  #include "intlist.h"
>  
>  /* use raw logging */
> @@ -294,7 +296,8 @@ cs_etm_decoder__do_soft_timestamp(struct cs_etm_queue *etmq,
>  static ocsd_datapath_resp_t
>  cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq,
>  				  const ocsd_generic_trace_elem *elem,
> -				  const uint8_t trace_chan_id)
> +				  const uint8_t trace_chan_id,
> +				  const ocsd_trc_index_t indx)
>  {
>  	struct cs_etm_packet_queue *packet_queue;
>  
> @@ -313,14 +316,33 @@ cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq,
>  		return OCSD_RESP_CONT;
>  	}
>  
> -	/*
> -	 * This is the first timestamp we've seen since the beginning of traces
> -	 * or a discontinuity.  Since timestamps packets are generated *after*
> -	 * range packets have been generated, we need to estimate the time at
> -	 * which instructions started by subtracting the number of instructions
> -	 * executed to the timestamp.
> -	 */
> -	packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
> +
> +	if (!elem->timestamp) {
> +		/*
> +		 * Zero timestamps can be seen due to misconfiguration or hardware bugs.
> +		 * Warn once, and don't try to subtract instr_count as it would result in an
> +		 * underflow.
> +		 */
> +		packet_queue->cs_timestamp = 0;
> +		WARN_ONCE(true, "Zero Coresight timestamp found at Idx:%" OCSD_TRC_IDX_STR
> +				". Decoding may be improved with --itrace=Z...\n", indx);
> +	} else if (packet_queue->instr_count > elem->timestamp) {
> +		/*
> +		 * Sanity check that the elem->timestamp - packet_queue->instr_count would not
> +		 * result in an underflow. Warn and clamp at 0 if it would.
> +		 */
> +		packet_queue->cs_timestamp = 0;
> +		pr_err("Timestamp calculation underflow at Idx:%" OCSD_TRC_IDX_STR "\n", indx);
> +	} else {
> +		/*
> +		 * This is the first timestamp we've seen since the beginning of traces
> +		 * or a discontinuity.  Since timestamps packets are generated *after*
> +		 * range packets have been generated, we need to estimate the time at
> +		 * which instructions started by subtracting the number of instructions
> +		 * executed to the timestamp.
> +		 */
> +		packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
> +	}
>  	packet_queue->next_cs_timestamp = elem->timestamp;
>  	packet_queue->instr_count = 0;
>  
> @@ -542,7 +564,7 @@ cs_etm_decoder__set_tid(struct cs_etm_queue *etmq,
>  
>  static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
>  				const void *context,
> -				const ocsd_trc_index_t indx __maybe_unused,
> +				const ocsd_trc_index_t indx,
>  				const u8 trace_chan_id __maybe_unused,
>  				const ocsd_generic_trace_elem *elem)
>  {
> @@ -579,7 +601,8 @@ static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
>  		break;
>  	case OCSD_GEN_TRC_ELEM_TIMESTAMP:
>  		resp = cs_etm_decoder__do_hard_timestamp(etmq, elem,
> -							 trace_chan_id);
> +							 trace_chan_id,
> +							 indx);
>  		break;
>  	case OCSD_GEN_TRC_ELEM_PE_CONTEXT:
>  		resp = cs_etm_decoder__set_tid(etmq, packet_queue,
> -- 
> 2.28.0
>