Re: [RFC PATCH v6 3/5] sched, tracing: reorganize fields of switch event struct

From: Peter Zijlstra
Date: Thu Aug 03 2023 - 05:52:14 EST

On Thu, Aug 03, 2023 at 05:18:26AM -0400, Steven Rostedt wrote:
> On Thu, 3 Aug 2023 04:33:50 -0400
> Ze Gao <zegao2021@xxxxxxxxx> wrote:
> > 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.