Re: [RFC PATCH 07/11] sched: Add proxy execution

[Joel Fernandes <joel@joelfernandes.org>]

Hello Dietmar,

On Fri, Nov 04, 2022 at 06:09:26PM +0100, Dietmar Eggemann wrote:
> On 31/10/2022 19:00, Joel Fernandes wrote:
> > On Mon, Oct 31, 2022 at 05:39:45PM +0100, Dietmar Eggemann wrote:
> >> On 29/10/2022 05:31, Joel Fernandes wrote:
> >>> Hello Dietmar,
> >>>
> >>>> On Oct 24, 2022, at 6:13 AM, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
> >>>>
> >>>> On 03/10/2022 23:44, Connor O'Brien wrote:
> >>>>> From: Peter Zijlstra <peterz@infradead.org>
> 
> [...]
> 
> >>>>> +    rq_unpin_lock(rq, rf);
> >>>>> +    raw_spin_rq_unlock(rq);
> >>>>
> >>>> Don't we run into rq_pin_lock()'s:
> >>>>
> >>>> SCHED_WARN_ON(rq->balance_callback && rq->balance_callback !=
> >>>> &balance_push_callback)
> >>>>
> >>>> by releasing rq lock between queue_balance_callback(, push_rt/dl_tasks)
> >>>> and __balance_callbacks()?
> >>>
> >>> Apologies, I’m a bit lost here. The code you are responding to inline does not call rq_pin_lock, it calls rq_unpin_lock.  So what scenario does the warning trigger according to you?
> >>
> >> True, but the code which sneaks in between proxy()'s
> >> raw_spin_rq_unlock(rq) and raw_spin_rq_lock(rq) does.
> >>
> > 
> > Got it now, thanks a lot for clarifying. Can this be fixed by do a
> > __balance_callbacks() at:
> 
> I tried the: 
> 
>    head = splice_balance_callbacks(rq)
>    task_rq_unlock(rq, p, &rf);
>    ...
>    balance_callbacks(rq, head);
> 
> separation known from __sched_setscheduler() in __schedule() (right
> after pick_next_task()) but it doesn't work. Lot of `BUG: scheduling
> while atomic:`

How about something like the following? This should exclude concurrent
balance callback queues from other CPUs and let us release the rq lock early
in proxy(). I ran locktorture with your diff to make writer threads RT, and I
cannot reproduce any crash with it:

---8<-----------------------

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: [PATCH] Exclude balance callback queuing during proxy's migrate

In commit 565790d28b1e ("sched: Fix balance_callback()"), it is clear that rq
lock needs to be held when __balance_callbacks() in schedule() is called.
However, it is possible that because rq lock is dropped in proxy(), another
CPU, say in __sched_setscheduler() can queue balancing callbacks and cause
issues.

To remedy this, exclude balance callback queuing on other CPUs, during the
proxy().

Reported-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/core.c  | 15 +++++++++++++++
 kernel/sched/sched.h |  3 +++
 2 files changed, 18 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 88a5fa34dc06..f1dac21fcd90 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6739,6 +6739,10 @@ proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
 		p->wake_cpu = wake_cpu;
 	}
 
+	// Prevent other CPUs from queuing balance callbacks while we migrate
+	// tasks in the migrate_list with the rq lock released.
+	raw_spin_lock(&rq->balance_lock);
+
 	rq_unpin_lock(rq, rf);
 	raw_spin_rq_unlock(rq);
 	raw_spin_rq_lock(that_rq);
@@ -6758,7 +6762,18 @@ proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
 	}
 
 	raw_spin_rq_unlock(that_rq);
+
+	// This may make lockdep unhappy as we acquire rq->lock with balance_lock
+	// held. But that should be a false positive, as the following pattern
+	// happens only on the current CPU with interrupts disabled:
+	// rq_lock()
+	// balance_lock();
+	// rq_unlock();
+	// rq_lock();
 	raw_spin_rq_lock(rq);
+
+	raw_spin_unlock(&rq->balance_lock);
+
 	rq_repin_lock(rq, rf);
 
 	return NULL; /* Retry task selection on _this_ CPU. */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 354e75587fed..932d32bf9571 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1057,6 +1057,7 @@ struct rq {
 	unsigned long		cpu_capacity_orig;
 
 	struct callback_head	*balance_callback;
+	raw_spinlock_t		balance_lock;
 
 	unsigned char		nohz_idle_balance;
 	unsigned char		idle_balance;
@@ -1748,6 +1749,7 @@ queue_balance_callback(struct rq *rq,
 		       void (*func)(struct rq *rq))
 {
 	lockdep_assert_rq_held(rq);
+	raw_spin_lock(&rq->balance_lock);
 
 	/*
 	 * Don't (re)queue an already queued item; nor queue anything when
@@ -1760,6 +1762,7 @@ queue_balance_callback(struct rq *rq,
 	head->func = (void (*)(struct callback_head *))func;
 	head->next = rq->balance_callback;
 	rq->balance_callback = head;
+	raw_spin_unlock(&rq->balance_lock);
 }
 
 #define rcu_dereference_check_sched_domain(p) \
-- 
2.38.1.584.g0f3c55d4c2-goog