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Date:   Thu, 21 May 2020 15:52:02 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        vpillai <vpillai@...italocean.com>, linux-kernel@...r.kernel.org,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, aubrey.li@...ux.intel.com,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [PATCH RFC] sched: Use sched-RCU in core-scheduling balancing
 logic

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@...italocean.com>
> Cc: Aaron Lu <aaron.lwe@...il.com>
> Cc: Aubrey Li <aubrey.intel@...il.com>
> Cc: peterz@...radead.org
> Cc: paulmck@...nel.org
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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...

						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
> 

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