Re: [PATCH v2 1/7] perf cs-etm: Don't flush when packet_queue fills up

[Leo Yan <leo.yan@arm.com>]

On 9/12/24 16:11, James Clark wrote:> 
> cs_etm__flush(), like cs_etm__sample() is an operation that generates a
> sample and then swaps the current with the previous packet. Calling
> flush after processing the queues results in two swaps which corrupts
> the next sample. Therefore it wasn't appropriate to call flush here so
> remove it.
> 
> Flushing is still done on a discontinuity to explicitly clear the last
> branch buffer, but when the packet_queue fills up before reaching a
> timestamp, that's not a discontinuity and the call to
> cs_etm__process_traceid_queue() already generated samples and drained
> the buffers correctly.
> 
> This is visible by looking for a branch that has the same target as the
> previous branch and the following source is before the address of the
> last target, which is impossible as execution would have had to have
> gone backwards:
> 
>    ffff800080849d40 _find_next_and_bit+0x78 => ffff80008011cadc update_sg_lb_stats+0x94
>     (packet_queue fills here before a timestamp, resulting in a flush and
>      branch target ffff80008011cadc is duplicated.)
>    ffff80008011cb1c update_sg_lb_stats+0xd4 => ffff80008011cadc update_sg_lb_stats+0x94
>    ffff8000801117c4 cpu_util+0x24 => ffff8000801117d4 cpu_util+0x34
> 
> After removing the flush the correct branch target is used for the
> second sample, and ffff8000801117c4 is no longer before the previous
> address:
> 
>    ffff800080849d40 _find_next_and_bit+0x78 => ffff80008011cadc update_sg_lb_stats+0x94
>    ffff80008011cb1c update_sg_lb_stats+0xd4 => ffff8000801117a0 cpu_util+0x0
>    ffff8000801117c4 cpu_util+0x24 => ffff8000801117d4 cpu_util+0x34
> 
> Make sure that a final branch stack is output at the end of the trace
> by calling cs_etm__end_block(). This is already done for both the
> timeless decode paths.

It is right to call cs_etm__flush() for only discontinuity packet and use
cs_etm__end_block() for flushing the end of data block. Thanks for
distinguishing these two different things.

> Fixes: 21fe8dc1191a ("perf cs-etm: Add support for CPU-wide trace scenarios")
> Reported-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
> Closes: https://lore.kernel.org/all/20240719092619.274730-1-gankulkarni@os.amperecomputing.com/
> Signed-off-by: James Clark <james.clark@linaro.org>

Reviewed-by: Leo Yan <leo.yan@arm.com>

> ---
>   tools/perf/util/cs-etm.c | 25 ++++++++++++++++++-------
>   1 file changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 90f32f327b9b..242788ac9625 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -2490,12 +2490,6 @@ static void cs_etm__clear_all_traceid_queues(struct cs_etm_queue *etmq)
> 
>                  /* Ignore return value */
>                  cs_etm__process_traceid_queue(etmq, tidq);
> -
> -               /*
> -                * Generate an instruction sample with the remaining
> -                * branchstack entries.
> -                */
> -               cs_etm__flush(etmq, tidq);
>          }
>   }
> 
> @@ -2638,7 +2632,7 @@ static int cs_etm__process_timestamped_queues(struct cs_etm_auxtrace *etm)
> 
>          while (1) {
>                  if (!etm->heap.heap_cnt)
> -                       goto out;
> +                       break;
> 
>                  /* Take the entry at the top of the min heap */
>                  cs_queue_nr = etm->heap.heap_array[0].queue_nr;
> @@ -2721,6 +2715,23 @@ static int cs_etm__process_timestamped_queues(struct cs_etm_auxtrace *etm)
>                  ret = auxtrace_heap__add(&etm->heap, cs_queue_nr, cs_timestamp);
>          }
> 
> +       for (i = 0; i < etm->queues.nr_queues; i++) {
> +               struct int_node *inode;
> +
> +               etmq = etm->queues.queue_array[i].priv;
> +               if (!etmq)
> +                       continue;
> +
> +               intlist__for_each_entry(inode, etmq->traceid_queues_list) {
> +                       int idx = (int)(intptr_t)inode->priv;
> +
> +                       /* Flush any remaining branch stack entries */
> +                       tidq = etmq->traceid_queues[idx];
> +                       ret = cs_etm__end_block(etmq, tidq);
> +                       if (ret)
> +                               return ret;
> +               }
> +       }
>   out:
>          return ret;
>   }
> --
> 2.34.1
>