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Message-ID: <20230803095132.GI212435@hirez.programming.kicks-ass.net>
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.
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