On Thu, Aug 03, 2023 at 05:18:26AM -0400, Steven Rostedt wrote:

On Thu,  3 Aug 2023 04:33:50 -0400
Ze Gao [off-list ref] wrote:

quoted

Report prioritiy and prev_state in 'short' to save some buffer
space. And also reorder the fields so that we take struct
alignment into consideration to make the record compact.

If I were to write this, I would have wrote:

  The prev_state field in the sched_switch event is assigned by
  __trace_sched_switch_state(). The largest number that function will return
  is TASK_REPORT_MAX which is just 0x100. There's no reason that the
  prev_state field is a full 32 bits when it is using just 9 bits max. In
  order to save space on the ring buffer, shrink the prev_state to 16 bits
  (short).

  Also, change the positions of the other fields to accommodate the short
  value of prev_state to eliminate any holes that were created in the
  structure.

See the difference?

This also doesn't mention you broke the data format for all trace events
a while back to ensure people are using libtracefs and are thus
confident this won't break things.