Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 20 Apr 2021 16:58:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Donnefort <vincent.donnefort@arm.com>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de,
        swood@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, qais.yousef@arm.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs
 hotplug-rollback
Message-ID: <YH7r+AoQEReSvxBI@hirez.programming.kicks-ass.net>
References: <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me>
 <YHQ3Iy7QfL+0UoM0@hirez.programming.kicks-ass.net>
 <87r1jfmn8d.mognet@arm.com>
 <YHU/a9HvGLYpOLKZ@hirez.programming.kicks-ass.net>
 <YHgAYef83VQhKdC2@hirez.programming.kicks-ass.net>
 <87a6pzmxec.mognet@arm.com>
 <20210419105541.GA40111@e120877-lin.cambridge.arm.com>
 <20210420094632.GA165360@e120877-lin.cambridge.arm.com>
 <YH7jSPZx0BhyHoLe@hirez.programming.kicks-ass.net>
 <YH7niBZDWjsz+jBa@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YH7niBZDWjsz+jBa@hirez.programming.kicks-ass.net>
Precedence: bulk

On Tue, Apr 20, 2021 at 04:39:04PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 04:20:56PM +0200, Peter Zijlstra wrote:
> > On Tue, Apr 20, 2021 at 10:46:33AM +0100, Vincent Donnefort wrote:
> > 
> > > Found the issue:
> > > 
> > > $ cat hotplug/states:
> > > 219: sched:active
> > > 220: online
> > > 
> > > CPU0: 
> > > 
> > > $ echo 219 > hotplug/fail
> > > $ echo 0 > online
> > > 
> > > => cpu_active = 1 cpu_dying = 1
> > > 
> > > which means that later on, for another CPU hotunplug, in
> > > __balance_push_cpu_stop(), the fallback rq for a kthread can select that
> > > CPU0, but __migrate_task() would fail and we end-up in an infinite loop,
> > > trying to migrate that task to CPU0.
> > > 
> > > The problem is that for a failure in sched:active, as "online" has no callback,
> > > there will be no call to cpuhp_invoke_callback(). Hence, the cpu_dying bit would
> > > not be reset.
> > 
> > Urgh! Good find.

> I seem to have triggered the BUG() in select_fallback_rq() with your recipie.
> Have cpu0 fail on sched:active, then offline all other CPUs.
> 
> Now lemme add that patch.

(which obviously didn't actually build) seems to fix it.

---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 838dcf238f92..e538518556f4 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -63,6 +63,7 @@ struct cpuhp_cpu_state {
 	bool			rollback;
 	bool			single;
 	bool			bringup;
+	int			cpu;
 	struct hlist_node	*node;
 	struct hlist_node	*last;
 	enum cpuhp_state	cb_state;
@@ -160,9 +161,6 @@ static int cpuhp_invoke_callback(unsigned int cpu, enum cpuhp_state state,
 	int (*cb)(unsigned int cpu);
 	int ret, cnt;
 
-	if (cpu_dying(cpu) != !bringup)
-		set_cpu_dying(cpu, !bringup);
-
 	if (st->fail == state) {
 		st->fail = CPUHP_INVALID;
 		return -EAGAIN;
@@ -467,13 +465,16 @@ static inline enum cpuhp_state
 cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
 {
 	enum cpuhp_state prev_state = st->state;
+	bool bringup = st->state < target;
 
 	st->rollback = false;
 	st->last = NULL;
 
 	st->target = target;
 	st->single = false;
-	st->bringup = st->state < target;
+	st->bringup = bringup;
+	if (cpu_dying(st->cpu) != !bringup)
+		set_cpu_dying(st->cpu, !bringup);
 
 	return prev_state;
 }
@@ -481,6 +482,8 @@ cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
 static inline void
 cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
 {
+	bool bringup = !st->bringup;
+
 	st->target = prev_state;
 
 	/*
@@ -503,7 +506,9 @@ cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
 			st->state++;
 	}
 
-	st->bringup = !st->bringup;
+	st->bringup = bringup;
+	if (cpu_dying(st->cpu) != !bringup)
+		set_cpu_dying(st->cpu, !bringup);
 }
 
 /* Regular hotplug invocation of the AP hotplug thread */
@@ -693,6 +698,7 @@ static void cpuhp_create(unsigned int cpu)
 
 	init_completion(&st->done_up);
 	init_completion(&st->done_down);
+	st->cpu = cpu;
 }
 
 static int cpuhp_should_run(unsigned int cpu)