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

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Mar 13, 2020 at 07:29:18PM -0400, Joel Fernandes (Google) wrote:
> rcu_read_unlock() can incur an infrequent deadlock in
> sched_core_balance(). Fix this by using the RCU-sched flavor 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>

The original could indeed deadlock, and this would avoid that deadlock.
(The commit to solve this deadlock is sadly not yet in mainline.)

Acked-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  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 3045bd50e249..037e8f2e2686 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4735,7 +4735,7 @@ 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))
> @@ -4748,7 +4748,7 @@ 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);
> -- 
> 2.25.1.481.gfbce0eb801-goog
>