linux-kernel - Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

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Date:   Tue, 25 Oct 2022 07:52:07 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, mingo@...nel.org,
        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, intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

On Fri, Oct 21, 2022 at 08:22:41PM +0300, Ville Syrjälä wrote:
> On Mon, Aug 22, 2022 at 01:18:22PM +0200, Peter Zijlstra wrote:
> > +#ifdef CONFIG_LOCKDEP
> > +	/*
> > +	 * It's dangerous to freeze with locks held; there be dragons there.
> > +	 */
> > +	if (!(state & __TASK_FREEZABLE_UNSAFE))
> > +		WARN_ON_ONCE(debug_locks && p->lockdep_depth);
> > +#endif
> 
> We now seem to be hitting this sporadically in the intel gfx CI.
> 
> I've spotted it on two machines so far:
> https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_12270/shard-tglb7/igt@gem_ctx_isolation@preservation-s3@vcs0.html
> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_109950v1/shard-snb5/igt@kms_flip@flip-vs-suspend-interruptible@a-vga1.html

Sadly no luck in reproducing this locally so far. In the meantime
I added the following patch into our topic/core-for-CI branch in
the hopes of CI stumbling on it again and dumping a bit more data:

--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -125,8 +125,16 @@ static int __set_task_frozen(struct task_struct *p, void *arg)
 	/*
 	 * It's dangerous to freeze with locks held; there be dragons there.
 	 */
-	if (!(state & __TASK_FREEZABLE_UNSAFE))
-		WARN_ON_ONCE(debug_locks && p->lockdep_depth);
+	if (!(state & __TASK_FREEZABLE_UNSAFE)) {
+		static bool warned = false;
+
+		if (!warned && debug_locks && p->lockdep_depth) {
+			debug_show_held_locks(p);
+			WARN(1, "%s/%d holding locks while freezing\n",
+			     p->comm, task_pid_nr(p));
+			warned = true;
+		}
+	}
 #endif
 
 	WRITE_ONCE(p->__state, TASK_FROZEN);
-- 
2.37.4

-- 
Ville Syrjälä
Intel