Re: [PATCH RFC] sched: Use sched-RCU in core-scheduling balancing logic

Joel Fernandes
2020-05-22T01:26:21Z

On Thu, May 21, 2020 at 6:52 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Wed, May 20, 2020 at 06:48:18PM -0400, Joel Fernandes (Google) wrote:
> > rcu_read_unlock() can incur an infrequent deadlock in
> > sched_core_balance(). Fix this by using sched-RCU instead.
> >
> > This fixes the following spinlock recursion observed when testing the
> > core scheduling patches on PREEMPT=y kernel on ChromeOS:
> >
> > [ 3.240891] BUG: spinlock recursion on CPU#2, swapper/2/0
> > [ 3.240900] lock: 0xffff9cd1eeb28e40, .magic: dead4ead, .owner: swapper/2/0, .owner_cpu: 2
> > [ 3.240905] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 5.4.22htcore #4
> > [ 3.240908] Hardware name: Google Eve/Eve, BIOS Google_Eve.9584.174.0 05/29/2018
> > [ 3.240910] Call Trace:
> > [ 3.240919] dump_stack+0x97/0xdb
> > [ 3.240924] ? spin_bug+0xa4/0xb1
> > [ 3.240927] do_raw_spin_lock+0x79/0x98
> > [ 3.240931] try_to_wake_up+0x367/0x61b
> > [ 3.240935] rcu_read_unlock_special+0xde/0x169
> > [ 3.240938] ? sched_core_balance+0xd9/0x11e
> > [ 3.240941] __rcu_read_unlock+0x48/0x4a
> > [ 3.240945] __balance_callback+0x50/0xa1
> > [ 3.240949] __schedule+0x55a/0x61e
> > [ 3.240952] schedule_idle+0x21/0x2d
> > [ 3.240956] do_idle+0x1d5/0x1f8
> > [ 3.240960] cpu_startup_entry+0x1d/0x1f
> > [ 3.240964] start_secondary+0x159/0x174
> > [ 3.240967] secondary_startup_64+0xa4/0xb0
> > [ 14.998590] watchdog: BUG: soft lockup - CPU#0 stuck for 11s! [kworker/0:10:965]
> >
> > Cc: vpillai <vpillai@digitalocean.com>
> > Cc: Aaron Lu <aaron.lwe@gmail.com>
> > Cc: Aubrey Li <aubrey.intel@gmail.com>
> > Cc: peterz@infradead.org
> > Cc: paulmck@kernel.org
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Change-Id: I1a4bf0cd1426b3c21ad5de44719813ad4ee5805e
>
> With some luck, the commit removing the need for this will hit
> mainline during the next merge window. Fingers firmly crossed...

Sounds good, thank you Paul :-)

- Joel

>
> Thanx, Paul
>
> > ---
> > kernel/sched/core.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 780514d03da47..b8ca6fcaaaf06 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4897,7 +4897,7 @@ static void sched_core_balance(struct rq *rq)
> > struct sched_domain *sd;
> > int cpu = cpu_of(rq);
> >
> > - rcu_read_lock();
> > + rcu_read_lock_sched();
> > raw_spin_unlock_irq(rq_lockp(rq));
> > for_each_domain(cpu, sd) {
> > if (!(sd->flags & SD_LOAD_BALANCE))
> > @@ -4910,7 +4910,7 @@ static void sched_core_balance(struct rq *rq)
> > break;
> > }
> > raw_spin_lock_irq(rq_lockp(rq));
> > - rcu_read_unlock();
> > + rcu_read_unlock_sched();
> > }
> >
> > static DEFINE_PER_CPU(struct callback_head, core_balance_head);
> > --
> > 2.26.2.761.g0e0b3e54be-goog
> >