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

[Joel Fernandes]

On Sun, Nov 20, 2022 at 7:22 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxx> wrote:
> > >>>>
> > >>>> On 03/10/2022 23:44, Connor O'Brien wrote:
> > >>>>> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >
> > [...]
> >
> > >>>>> +    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@xxxxxxxxxxxxxxxxx>
> 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@xxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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);

Hmm, I think there's still a chance of deadlock here. I need to
rethink it a bit, but that's the idea I was going for.

thanks,

 - Joel


> +
> +       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
>