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Date: Wed, 29 Apr 2015 15:26:58 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Don Zickus <dzickus@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Andrew Jones <drjones@redhat.com>,
        chai wen <chaiw.fnst@cn.fujitsu.com>,
        Ulrich Obergfell <uobergfe@redhat.com>,
        Fabian Frederick <fabf@skynet.be>, Aaron Tomlin <atomlin@redhat.com>,
        Ben Zhang <benzh@chromium.org>, "Christoph Lameter" <cl@linux.com>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        "Steven Rostedt" <rostedt@goodmis.org>, <linux-kernel@vger.kernel.org>,
        "Jonathan Corbet" <corbet@lwn.net>, <linux-doc@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v9 3/3] procfs: treat parked tasks as sleeping for task
 state
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On Fri, 17 Apr 2015 14:37:18 -0400 Chris Metcalf <cmetcalf@ezchip.com> wrote:

> Allowing watchdog threads to be parked means that we now have the
> opportunity of actually seeing persistent parked threads in the output
> of /proc's stat and status files.  The existing code reported such

"/proc's stat" is ambiguous (/proc/stat?).  We can remove all doubt by
using full pathnames: /proc/<pid>/stat.

> threads as "Running", which is kind-of true if you think of the case
> where we park them as part of taking cpus offline.  But if we allow
> parking them indefinitely, "Running" is pretty misleading, so we report
> them as "Sleeping" instead.
> 
> We could simply report them with a new string, "Parked", but it feels
> like it's a bit risky for userspace to see unexpected new values.
> The scheduler does report parked tasks with a "P" in debugging output
> from sched_show_task() or dump_cpu_task(), but that's a different API.
> 
> This change seemed slightly cleaner than updating the task_state_array
> to have additional rows.  TASK_DEAD should be subsumed by the exit_state
> bits; TASK_WAKEKILL is just a modifier; and TASK_WAKING can very
> reasonably be reported as "Running" (as it is now).  Only TASK_PARKED
> shows up with unreasonable output here.

Documentation/filesystems/proc.txt documents /proc/<pid>/status.  It
documents "State" explicitly.

> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -126,6 +126,10 @@ static inline const char *get_task_state(struct task_struct *tsk)
>  {
>  	unsigned int state = (tsk->state | tsk->exit_state) & TASK_REPORT;
>  
> +	/* Treat parked tasks as sleeping. */
> +	if (tsk->state == TASK_PARKED)
> +		state = TASK_INTERRUPTIBLE;

The comment describes something which is utterly obvious.  What it
doesn't describe (and should) is *why* we do this.

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