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

[Li, Aubrey]

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?

Thanks,
-Aubrey

[  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:

[  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
>>