linux-kernel - Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match

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Date:   Wed, 7 Sep 2022 11:30:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        ebiederm@...ssion.com, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match_state

On Tue, Sep 06, 2022 at 12:54:34PM +0200, Peter Zijlstra wrote:

> > Suggestion #3:
> > 
> > - Couldn't the following users with a 0 mask:
> > 
> >     drivers/powercap/idle_inject.c:         wait_task_inactive(iit->tsk, 0);
> >     fs/coredump.c:                  wait_task_inactive(ptr->task, 0);
> > 
> >   Use ~0 instead (exposed as TASK_ANY or so) and then we can drop the
> >   !match_state special case?
> > 
> >   They'd do something like:
> > 
> >     drivers/powercap/idle_inject.c:         wait_task_inactive(iit->tsk, TASK_ANY);
> >     fs/coredump.c:                  wait_task_inactive(ptr->task, TASK_ANY);
> > 
> >   It's not an entirely 100% equivalent transformation though, but looks OK 
> >   at first sight: ->__state will be some nonzero mask for genuine tasks 
> >   waiting to schedule out, so any match will be functionally the same as a 
> >   0 flag telling us not to check any of the bits, right? I might be missing 
> >   something though.
> 
> I too am thinking that should work. Added patch for that.

---
Subject: sched: Add TASK_ANY for wait_task_inactive()
From: Peter Zijlstra <peterz@...radead.org>
Date: Tue Sep 6 12:39:55 CEST 2022

Now that wait_task_inactive()'s @match_state argument is a mask (like
ttwu()) it is possible to replace the special !match_state case with
an 'all-states' value such that any blocked state will match.

Suggested-by: Ingo Molnar (mingo@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 drivers/powercap/idle_inject.c |    2 +-
 fs/coredump.c                  |    2 +-
 include/linux/sched.h          |    2 ++
 kernel/sched/core.c            |   16 ++++++++--------
 4 files changed, 12 insertions(+), 10 deletions(-)

--- a/drivers/powercap/idle_inject.c
+++ b/drivers/powercap/idle_inject.c
@@ -254,7 +254,7 @@ void idle_inject_stop(struct idle_inject
 		iit = per_cpu_ptr(&idle_inject_thread, cpu);
 		iit->should_run = 0;
 
-		wait_task_inactive(iit->tsk, 0);
+		wait_task_inactive(iit->tsk, TASK_ANY);
 	}
 
 	cpu_hotplug_enable();
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -412,7 +412,7 @@ static int coredump_wait(int exit_code,
 		 */
 		ptr = core_state->dumper.next;
 		while (ptr != NULL) {
-			wait_task_inactive(ptr->task, 0);
+			wait_task_inactive(ptr->task, TASK_ANY);
 			ptr = ptr->next;
 		}
 	}
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -101,6 +101,8 @@ struct task_group;
 #define TASK_RTLOCK_WAIT		0x1000
 #define TASK_STATE_MAX			0x2000
 
+#define TASK_ANY			(TASK_STATE_MAX-1)
+
 /* Convenience macros for the sake of set_current_state: */
 #define TASK_KILLABLE			(TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
 #define TASK_STOPPED			(TASK_WAKEKILL | __TASK_STOPPED)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3254,12 +3254,12 @@ int migrate_swap(struct task_struct *cur
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
- * If @match_state is nonzero, it's the @p->state value just checked and
- * not expected to change.  If it changes, i.e. @p might have woken up,
- * then return zero.  When we succeed in waiting for @p to be off its CPU,
- * we return a positive number (its total switch count).  If a second call
- * a short while later returns the same number, the caller can be sure that
- * @p has remained unscheduled the whole time.
+ * Wait for the thread to block in any of the states set in @match_state.
+ * If it changes, i.e. @p might have woken up, then return zero.  When we
+ * succeed in waiting for @p to be off its CPU, we return a positive number
+ * (its total switch count).  If a second call a short while later returns the
+ * same number, the caller can be sure that @p has remained unscheduled the
+ * whole time.
  *
  * The caller must ensure that the task *will* unschedule sometime soon,
  * else this function might spin for a *long* time. This function can't
@@ -3295,7 +3295,7 @@ unsigned long wait_task_inactive(struct
 		 * is actually now running somewhere else!
 		 */
 		while (task_on_cpu(rq, p)) {
-			if (match_state && !(READ_ONCE(p->__state) & match_state))
+			if (!(READ_ONCE(p->__state) & match_state))
 				return 0;
 			cpu_relax();
 		}
@@ -3310,7 +3310,7 @@ unsigned long wait_task_inactive(struct
 		running = task_on_cpu(rq, p);
 		queued = task_on_rq_queued(p);
 		ncsw = 0;
-		if (!match_state || (READ_ONCE(p->__state) & match_state))
+		if (READ_ONCE(p->__state) & match_state)
 			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
 		task_rq_unlock(rq, p, &rf);