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Date:   Thu, 21 May 2020 21:26:05 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     paulmck@...nel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        vpillai <vpillai@...italocean.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Frederic Weisbecker" <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <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>
Subject: Re: [PATCH RFC] sched: Use sched-RCU in core-scheduling balancing logic

On Thu, May 21, 2020 at 6:52 PM Paul E. McKenney <paulmck@...nel.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@...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...

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
> >

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