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From:   Denis Nikitin <denik@google.com>
Date:   Tue, 11 May 2021 01:06:03 -0700
Message-ID: <CAOYpmdFYs8=FzpiA-mMNZ=L8B9GRXLfQuEnMeDsvWeeKg2PWtA@mail.gmail.com>
Subject: Re: [RFC PATCH] perf cs-etm: Handle valid-but-zero timestamps
To:     Leo Yan <leo.yan@linaro.org>
Cc:     James Clark <james.clark@arm.com>, coresight@lists.linaro.org,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Al Grant <al.grant@arm.com>,
        Branislav Rankov <branislav.rankov@arm.com>,
        Denis Nikitin <denik@chromium.org>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        anshuman.khandual@arm.com, Mike Leach <mike.leach@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        John Garry <john.garry@huawei.com>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
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Hi Leo,


> Just remind, as Mike has mentioned that if the timestamp is zero, it
> means the hardware setting for timestamp is not enabled properly.  So
> for system wide or per CPU mode tracing, it's better to double check
> what's the reason the timestamp is not enabled properly.

The bug is confirmed by HW verification.

>
> IIUC, this patch breaks the existed rational in the code.  Let's think
> about there have 4 CPUs, every CPU has its own AUX trace buffer, and
> when decode the trace data, it will use 4 queues to track the packets
> and every queue has its timestamp.
>
>   CPU0: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU1: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU2: cs_etm_queue -> ... -> packet_queue->timestamp
>   CPU3: cs_etm_queue -> ... -> packet_queue->timestamp
>
> The issue is if all CPUs' timestamp are zero, it's impossible to find
> a way to synthesize samples in the right time order.

Is it really impossible or it just can lead to incorrect decoding?
I verified the profiles generated with zero timestamps and this patch
on Trogdor (8 CPU cores) and I don't see any significant difference
in the quality of the AutoFDO profiles.

If mixed packets don't cause errors in reconstructing the branches
but instead mess up with their timeline then it probably won't matter
for AutoFDO which just collects statistics of the branches.
What do you think?

>
> [...]
>
> Thanks,
> Leo

Thanks,
Denis