On 8/04/21 6:31 pm, Alexander Shishkin wrote:
> Turns out, the default setting of attr.aux_watermark to half of the total
> buffer size is not very useful, especially with smaller buffers. The
> problem is that, after half of the buffer is filled up, the kernel updates
> ->aux_head and sets up the next "transaction", while observing that
> ->aux_tail is still zero (as userspace haven't had the chance to update
> it), meaning that the trace will have to stop at the end of this second
> "transaction". This means, for example, that the second PERF_RECORD_AUX in
> every trace comes with TRUNCATED flag set.
>
> Setting attr.aux_watermark to quarter of the buffer gives enough space for
> the ->aux_tail update to be observed and prevents the data loss.
>
> The obligatory before/after showcase:
>
>> # perf_before record -e intel_pt//u -m,8 uname
>> Linux
>> [ perf record: Woken up 6 times to write data ]
>> Warning:
>> AUX data lost 4 times out of 10!
>>
>> [ perf record: Captured and wrote 0.099 MB perf.data ]
>> # perf record -e intel_pt//u -m,8 uname
>> Linux
>> [ perf record: Woken up 4 times to write data ]
>> [ perf record: Captured and wrote 0.039 MB perf.data ]
>
> The effect is still visible with large workloads and large buffers,
> although less pronounced.
>
> Signed-off-by: Alexander Shishkin <alexander.shish...@linux.intel.com>
> ---
> tools/perf/arch/x86/util/intel-pt.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/tools/perf/arch/x86/util/intel-pt.c
> b/tools/perf/arch/x86/util/intel-pt.c
> index a6420c647959..d00707faf547 100644
> --- a/tools/perf/arch/x86/util/intel-pt.c
> +++ b/tools/perf/arch/x86/util/intel-pt.c
> @@ -776,6 +776,10 @@ static int intel_pt_recording_options(struct
> auxtrace_record *itr,
> }
> }
>
> + if (opts->full_auxtrace)
> + intel_pt_evsel->core.attr.aux_watermark =
> + opts->auxtrace_mmap_pages / 4 * page_size;
> +
I would be explicit about the mode and put "/ 4" at the end
for the case auxtrace_mmap_pages is not a multiple of 4 (e.g. 2).
i.e.
if (!opts->auxtrace_snapshot_mode && !opts->auxtrace_sample_mode) {
u32 aux_watermark = opts->auxtrace_mmap_pages * page_size / 4;
intel_pt_evsel->core.attr.aux_watermark = aux_watermark;
}
> intel_pt_parse_terms(intel_pt_pmu->name, &intel_pt_pmu->format,
> "tsc", &tsc_bit);
>
>
