2016-10-09 1:06 GMT+08:00 Peter Zijlstra <pet...@infradead.org>:
> On Sat, Oct 08, 2016 at 06:24:38PM +0800, Wanpeng Li wrote:
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 543b2f2..03a6620 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5472,19 +5472,29 @@ static inline int select_idle_smt(struct task_struct
>> *p, struct sched_domain *sd
>> */
>> static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd,
>> int target)
>> {
>> - struct sched_domain *this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>
> So select_idle_cpu() <- select_idle_sibling() is called from two places,
> both which already hold rcu_read_lock() afaict.
Agreed.
>
> This would've insta-triggered a rcu-lockdep splat otherwise I think.
CONFIG_LOCKDEP_SUPPORT=y
CONFIG_LOCKDEP=y
CONFIG_DEBUG_LOCKDEP=y
CONFIG_DEBUG_LOCK_ALLOC=y
So it is interesting why not a rcu-lockdep splat. :)
>
> That is, selsect_task_rq_fair() has rcu_read_lock() taken when calling
> this, and task_numa_compare() does too.
>
>> + struct sched_domain *this_sd;
>> u64 avg_idle = this_rq()->avg_idle;
>> - u64 avg_cost = this_sd->avg_scan_cost;
>> + u64 avg_cost;
>> u64 time, cost;
>> s64 delta;
>> int cpu, wrap;
>>
>> + rcu_read_lock();
>> + this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>> + if (!this_sd) {
>> + cpu = -1;
>> + goto unlock;
>> + }
>
> Yes, this is the part that was missing. We need to test this_sd after
> the lookup.
>
> Thanks for looking at this!
NP, I will send out v2 soon. :)
Regards,
Wanpeng Li
