Re: [PATCH v2 2/3] perf cs-etm: Remove callback cs_etm_find_snapshot()

From: Arnaldo Carvalho de Melo
Date: Thu Jul 01 2021 - 12:25:20 EST

Em Thu, Jul 01, 2021 at 05:35:36PM +0800, Leo Yan escreveu:
> The callback cs_etm_find_snapshot() is invoked for snapshot mode, its
> main purpose is to find the correct AUX trace data and returns "head"
> and "old" (we can call "old" as "old head") to the caller, the caller
> __auxtrace_mmap__read() uses these two pointers to decide the AUX trace
> data size.
> This patch removes cs_etm_find_snapshot() with below reasons:
> - The first thing in cs_etm_find_snapshot() is to check if the head has
> wrapped around, if it is not, directly bails out. The checking is
> pointless, this is because the "head" and "old" pointers both are
> monotonical increasing so they never wrap around.
> - cs_etm_find_snapshot() adjusts the "head" and "old" pointers and
> assumes the AUX ring buffer is fully filled with the hardware trace
> data, so it always subtracts the difference "mm->len" from "head" to
> get "old". Let's imagine the snapshot is taken in very short
> interval, the tracers only fill a small chunk of the trace data into
> the AUX ring buffer, in this case, it's wrongly to copy the whole the
> AUX ring buffer to perf file.
> - As the "head" and "old" pointers are monotonically increased, the
> function __auxtrace_mmap__read() handles these two pointers properly.
> It calculates the reminders for these two pointers, and the size is
> clamped to be never more than "snapshot_size". We can simply reply on
> the function __auxtrace_mmap__read() to calculate the correct result
> for data copying, it's not necessary to add Arm CoreSight specific
> callback.

Thanks, applied.

- Arnaldo