Re: [PATCH] perf cs-etm: Fix kernel timestamp handling

On 12/09/2023 02:52, Leo Yan wrote:

Hi James,

On Mon, Sep 11, 2023 at 02:24:09PM +0100, James Clark wrote:

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The current code can support both timestamp sources when synthesizing samples. However, the decoding flow only relies on the hardware timestamp. If the hardware timestamp is zero, it becomes impossible to decode the trace data. Consequently, in this case, the commands below won't output any samples:

perf record -e cs_etm// --per-thread --timestamp -- ls
perf script

Hi Leo,

I couldn't reproduce this issue, even when hard coding the hardware timestamp to zero in cs_etm_decoder__do_hard_timestamp() like this:

converted_timestamp = 0;

I reproduced this issue on the Juno-r2 board, its "etm->has_virtual_ts" is always false and Arm CoreSight timestamp packets are zeros.

Besides set "converted_timestamp = 0", it might need to hard code "etm->has_virtual_ts" to false?

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I'm not sure why this would result in no samples being generated either,

because we don't actually use the timestamps for anything yet [1]. We always wait until the very end of the file before decoding to ensure that all of the mmaps are loaded. And the timestamp is just assigned to the samples, but they shouldn't affect whether they are generated or not.

Unless there is something else I'm missing?

Let's review below code.

cs_etm__queue_first_cs_timestamp() retrieves trace data and decodes it, and breaks the while loop until it find the timestamp is not zero or no trace data is avaliable. When the timestamp is always zero, the while loop continues to drop the CoreSight trace data and don't synthesize samples.

cs_etm__queue_first_cs_timestamp() { ...

  while(1) {
      ret = cs_etm__get_data_block(etmq);
      if (ret <= 0)
          goto out;

      ret = cs_etm__decode_data_block(etmq);
      if (ret)
          goto out;

      cs_timestamp = cs_etm__etmq_get_timestamp(etmq, &trace_chan_id);
      /* We found a timestamp, no need to continue. */
      if (cs_timestamp)
          break;

      cs_etm__clear_all_packet_queues(etmq);
  }

}

Ah, I couldn't reproduce it because I was missing your other patch to add Coresight timestamps when Perf timestamps are requested. If I add that patch or force Coresight timestamps then I can reproduce the issue:

$ perf record -e cs_etm/timestamp=1/ --per-thread --timestamp -- ls

But when running perf script I get this warning:

Zero Coresight timestamp found at Idx:39. Decoding may be improved by prepending 'Z' to your current --itrace arguments.

And then if I force timeless mode like it says to do I get samples:

$ perf script --itrace=Zi1000i

I'm not sure if silently making it use the kernel timestamp for sorting makes sense, especially when zero hardware timestamps are a bug, and we already have a warning and a workaround for it.

Also it would mean that you are requesting timestamps on a platform that isn't generating them. Maybe the fix could also just be to not request timestamps, and then it uses the timeless decoding flow that also works:

$ perf record -e cs_etm/timestamp=0/ --per-thread --timestamp -- ls

My worry is that mixing kernel and coresight timestamps for sorting could make the code confusing to reason about, and sorting based on a single static AUX timestamp doesn't make sense logically (it's almost like a hack where the side effect is to make it work). And there isn't really anything that there isn't already a workaround for. Unless you want to explicitly disable the "use the Z option" warning and make it work transparently? Which I'm not sure is the right thing to do.

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Also, in cs_etm__queue_first_cs_timestamp(), cs_timestamp is used for sorting the decoding order between CPUs, but if the hardware timestamp is 0, then it's 0 on all trace.

Correct.

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So the sorting would be the same if you change that to be the kernel timestamp. They're all still the same static number, but just a different number (because we wait until the end of the file, 'latest_kernel_timestamp' is always the timestamp of the last AUX event in the file).

If we use the 'latest_kernel_timestamp' as timestamp, it's non-zero timestamp rather than all timestamp '0'. Yes, 'latest_kernel_timestamp' is a coarse kernel timestamp which is shared by all trace data recorded in the AUX event, though it's a static number, it can allow us to break the while loop mentioned above.

I understand 'latest_kernel_timestamp' is inaccurate for sorting, but as least now it exists in current code, quotes from util/cs-etm.c:

/*

• Record the latest kernel timestamp available in the header
• for samples so that synthesised samples occur from this point
• onwards.

*/ if (sample->time && (sample->time != (u64)-1)) etm->latest_kernel_timestamp = sample->time;

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[1]: I still plan to change the decoding to decode up to the current time in the file, rather than waiting for the end of the file before starting. That way decoding trace when there were overlapping mmap regions in time will be more accurate.

Thanks for heading up. I am not sure if I understand this correctly, but seems to me it is a good thing to try for using overlapping mmap events.

Just a side topic, if an Arm platform connect the Arm timer counter to Arm CoreSight, and detect virtual timestamp is false (thus etm->has_virtual_ts is 0). I think this might happen on some legacy platforms, can we use some ways to allow users to still use the Arm CoreSight timestamp in this case? E.g. we can force set etm->has_virtual_ts to 1 when user specify the config '-e cs_etm/timestamp/'.

Thanks, Leo