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SubjectRe: [RESEND PATCH] perf intel-pt: Fix TSC slip
Em Mon, Mar 25, 2019 at 03:51:35PM +0200, Adrian Hunter escreveu:
> A TSC packet can slip past MTC packets so that the timestamp appears to go
> backwards. One estimate is that can be up to about 40 CPU cycles, which is
> certainly less than 0x1000 TSC ticks, but accept slippage an order of
> magnitude more to be on the safe side.

thanks, applied to perf/urgent.

- Arnaldo

> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> Fixes: 79b58424b821c ("perf tools: Add Intel PT support for decoding MTC packets")
> Cc: stable@vger.kernel.org
> ---
> .../util/intel-pt-decoder/intel-pt-decoder.c | 20 ++++++++-----------
> 1 file changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
> index 6e03db142091..872fab163585 100644
> --- a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
> +++ b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
> @@ -251,19 +251,15 @@ struct intel_pt_decoder *intel_pt_decoder_new(struct intel_pt_params *params)
> if (!(decoder->tsc_ctc_ratio_n % decoder->tsc_ctc_ratio_d))
> decoder->tsc_ctc_mult = decoder->tsc_ctc_ratio_n /
> decoder->tsc_ctc_ratio_d;
> -
> - /*
> - * Allow for timestamps appearing to backwards because a TSC
> - * packet has slipped past a MTC packet, so allow 2 MTC ticks
> - * or ...
> - */
> - decoder->tsc_slip = multdiv(2 << decoder->mtc_shift,
> - decoder->tsc_ctc_ratio_n,
> - decoder->tsc_ctc_ratio_d);
> }
> - /* ... or 0x100 paranoia */
> - if (decoder->tsc_slip < 0x100)
> - decoder->tsc_slip = 0x100;
> +
> + /*
> + * A TSC packet can slip past MTC packets so that the timestamp appears
> + * to go backwards. One estimate is that can be up to about 40 CPU
> + * cycles, which is certainly less than 0x1000 TSC ticks, but accept
> + * slippage an order of magnitude more to be on the safe side.
> + */
> + decoder->tsc_slip = 0x10000;
>
> intel_pt_log("timestamp: mtc_shift %u\n", decoder->mtc_shift);
> intel_pt_log("timestamp: tsc_ctc_ratio_n %u\n", decoder->tsc_ctc_ratio_n);
> --
> 2.17.1

--

- Arnaldo

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