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

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Date:   Thu, 3 Aug 2023 11:51:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ze Gao <zegao2021@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, Ze Gao <zegao@...cent.com>
Subject: Re: [RFC PATCH v6 3/5] sched, tracing: reorganize fields of switch
 event struct

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@...il.com> 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.