[PATCH v10 3/3] procfs: treat parked tasks as sleeping for task state

[Chris Metcalf]

Allowing watchdog threads to be parked means that we now have the
opportunity of actually seeing persistent parked threads in the output
of /proc/<pid>/stat and /proc/<pid>/status.  The existing code reported
such 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
output is already documented in Documentation/filesystems/proc.txt,
and it seems like a mistake to change that lightly.

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.
Similarly, the trace_ctxwake_* routines report a "P" for parked tasks,
but again, 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.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
 fs/proc/array.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index fd02a9ebfc30..3f57dac31ba6 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -126,6 +126,14 @@ static inline const char *get_task_state(struct task_struct *tsk)
 {
 	unsigned int state = (tsk->state | tsk->exit_state) & TASK_REPORT;
 
+	/*
+	 * Parked tasks do not run; they sit in __kthread_parkme().
+	 * Without this check, we would report them as running, which is
+	 * clearly wrong, so we report them as sleeping instead.
+	 */
+	if (tsk->state == TASK_PARKED)
+		state = TASK_INTERRUPTIBLE;
+
 	BUILD_BUG_ON(1 + ilog2(TASK_REPORT) != ARRAY_SIZE(task_state_array)-1);
 
 	return task_state_array[fls(state)];
-- 
2.1.2

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