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

[Li, Aubrey]

On 2020/3/25 2:49, Joel Fernandes wrote:
> On Tue, Mar 24, 2020 at 11:01:27AM +0800, Li, Aubrey wrote:
>> On 2020/3/23 23:21, Joel Fernandes wrote:
>>> 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?
>>>
>>
>> How about this?
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index a01df3e..7ff694e 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -4743,7 +4743,6 @@ static void sched_core_balance(struct rq *rq)
>>  	int cpu = cpu_of(rq);
>>  
>>  	rcu_read_lock();
>> -	raw_spin_unlock_irq(rq_lockp(rq));
>>  	for_each_domain(cpu, sd) {
>>  		if (!(sd->flags & SD_LOAD_BALANCE))
>>  			break;
>> @@ -4754,7 +4753,6 @@ static void sched_core_balance(struct rq *rq)
>>  		if (steal_cookie_task(cpu, sd))
>>  			break;
>>  	}
>> -	raw_spin_lock_irq(rq_lockp(rq));
> 
> try_steal_cookie() does a double_rq_lock(). Would this change not deadlock
> with that?
> 
Oh yes, missed double_rq_lock() inside, just want to keep local intr disabled
to avoid preemption. will try Paul's patch and report back.

Thanks,
-Aubrey