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From:   Ze Gao <zegao2021@gmail.com>
Date:   Mon, 31 Jul 2023 19:00:00 +0800
Message-ID: <CAD8CoPDvXN6u8uw48UU97bWJgkB0S2qGimAFwhHo9XvWe_x03A@mail.gmail.com>
Subject: Re: [RFC PATCH v2 1/3] sched, tracing: add to report task state in
 symbolic chars
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        linux-trace-kernel@vger.kernel.org,
        linux-trace-devel@vger.kernel.org, Ze Gao <zegao@tencent.com>
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On Mon, Jul 31, 2023 at 5:37=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Wed, Jul 26, 2023 at 08:16:16PM +0800, Ze Gao wrote:
> > Internal representations of task state are likely to be changed or
> > ordered, and reporting them to userspace without exporting them as
> > part of API is not a good choice, which can easily break a userspace
> > observability tool as kernel evolves. For example, perf suffers from
> > this and still reports wrong states by this patch.
> >
> > OTOH, some masqueraded state like TASK_REPORT_IDLE and TASK_REPORT_MAX
> > are also reported inadvertently, which confuses things even more.
> >
> > So add a new variable in company with the old raw value to report task
> > state in symbolic char, which is self-explaining and no further
> > translation is needed, and also report priorities in 'short' to save
> > some buffer space.  Of course this does not break any userspace tool.
> >
> > Note for PREEMPT_ACTIVE, we introduce 'p' to report it and use the old
> > conventions for the rest.
>
> So I really dont much like this. This looses the ability to see the
> actual wait state flags, there could be multiple. Eg, things like
> TASK_FREEZEABLE gets lost completely.

Also, IIRC, TASK_FREEZABLE which is defined as 0x2000, is already lost
in the current implementation of __trace_sched_switch_state which limits
all states except PREEMPT_ACTIIVE below TASK_REPORT_IDLE to be
reported. So I do not believe you can achieve this by just leaving things a=
lone.

Regards,
Ze