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

[Joel Fernandes]

On Mon, Mar 23, 2020 at 02:58:18PM +0800, Li, Aubrey wrote:
> On 2020/3/14 8:30, Paul E. McKenney wrote:
> > 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:
> >>
> >> [   14.998590] watchdog: BUG: soft lockup - CPU#0 stuck for 11s! [kworker/0:10:965]
> >>
> > 
> > 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@xxxxxxxxxx>
> 
> I saw this in dmesg with this patch, is it expected?
> 
> [  117.000905] =============================
> [  117.000907] WARNING: suspicious RCU usage
> [  117.000911] 5.5.7+ #160 Not tainted
> [  117.000913] -----------------------------
> [  117.000916] kernel/sched/core.c:4747 suspicious rcu_dereference_check() usage!
> [  117.000918] 
>                other info that might help us debug this:

Sigh, this is because for_each_domain() expects rcu_read_lock(). From an RCU
PoV, the code is correct (warning doesn't cause any issue).

To silence warning, we could replace the rcu_read_lock_sched() in my patch with:
preempt_disable();
rcu_read_lock();

and replace the unlock with:

rcu_read_unlock();
preempt_enable();

That should both take care of both the warning and the scheduler-related
deadlock. Thoughts?

Does that fix the warning for you? 

thanks,

 - Joel

> 
> [  117.000921] 
>                rcu_scheduler_active = 2, debug_locks = 1
> [  117.000923] 1 lock held by swapper/52/0:
> [  117.000925]  #0: ffffffff82670960 (rcu_read_lock_sched){....}, at: sched_core_balance+0x5/0x700
> [  117.000937] 
>                stack backtrace:
> [  117.000940] CPU: 52 PID: 0 Comm: swapper/52 Kdump: loaded Not tainted 5.5.7+ #160
> [  117.000943] Hardware name: Intel Corporation S2600WFD/S2600WFD, BIOS SE5C620.86B.01.00.0412.020920172159 02/09/2017
> [  117.000945] Call Trace:
> [  117.000955]  dump_stack+0x86/0xcb
> [  117.000962]  sched_core_balance+0x634/0x700
> [  117.000982]  __balance_callback+0x49/0xa0
> [  117.000990]  __schedule+0x1416/0x1620
> [  117.001000]  ? lockdep_hardirqs_off+0xa0/0xe0
> [  117.001005]  ? _raw_spin_unlock_irqrestore+0x41/0x70
> [  117.001024]  schedule_idle+0x28/0x40
> [  117.001030]  do_idle+0x17e/0x2a0
> [  117.001041]  cpu_startup_entry+0x19/0x20
> [  117.001048]  start_secondary+0x16c/0x1c0
> [  117.001055]  secondary_startup_64+0xa4/0xb0
> 
> > 
> >> ---
> >>  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
> >>
>