Hi Leo,

On 17/10/2021 07:13, Leo Yan wrote:

[...]

I looked into the Arm SPE driver and found it doesn't really support
free run mode for AUX ring buffer when the driver runs in snapshot
mode, the pair functions perf_aux_output_end() and
perf_aux_output_begin() are invoked when every time handle the
interrupt.  The detailed flow is:

  arm_spe_pmu_irq_handler()
    `> arm_spe_pmu_buf_get_fault_act()
         `> arm_spe_perf_aux_output_end()
              `> set SPE registers
              `> perf_aux_output_end()
    `> arm_spe_perf_aux_output_begin()
         `> perf_aux_output_begin()
         `> set SPE registers

Seems to me, a possible solution is to add an extra parameter 'int
in_interrupt' for functions arm_spe_perf_aux_output_end() and
arm_spe_perf_aux_output_begin(), if this parameter is passed as 1 in
the interrupt handling, these two functions should skip invoking
perf_aux_output_end() and perf_aux_output_begin() so can avoid the
redundant perf event PERF_RECORD_AUX.

  arm_spe_pmu_irq_handler()
    `> arm_spe_pmu_buf_get_fault_act()
         `> arm_spe_perf_aux_output_end(..., in_interrupt=1)
              `> set SPE registers
    `> arm_spe_perf_aux_output_begin(..., in_interrupt=1)
         `> set SPE registers

I brought the issue of the redundant AUX events to the team, and we know
of at least one tool in Arm relying on these events in snapshot mode. So
we think that changing this behavior of the driver might not be easy to
do right now.

P.s. I think Intel-PT has supported free run mode for snapshot mode,
so it should not generate interrupt in this mode.  Thus Intel-PT can
avoid this issue, please see the code [2].

Thanks,
Leo

[1] https://people.linaro.org/~leo.yan/spe/snapshot_test/perf.data
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/intel/pt.c#n753

Thanks,
German

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