Re: [PATCH 2/2] sched/fair: Fix use of NULL with find_idlest_group

[Brendan Jackman <brendan.jackman@arm.com>]

On Tue, Aug 22 2017 at 04:34, Joel Fernandes wrote:
> Hi Peter,
>
> On Mon, Aug 21, 2017 at 2:14 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> On Mon, Aug 21, 2017 at 04:21:28PM +0100, Brendan Jackman wrote:
>>> The current use of returning NULL from find_idlest_group is broken in
>>> two cases:
>>>
>>> a1) The local group is not allowed.
>>>
>>>    In this case, we currently do not change this_runnable_load or
>>>    this_avg_load from its initial value of 0, which means we return
>>>    NULL regardless of the load of the other, allowed groups. This
>>>    results in pointlessly continuing the find_idlest_group search
>>>    within the local group and then returning prev_cpu from
>>>    select_task_rq_fair.
>>
>>> b) smp_processor_id() is the "idlest" and != prev_cpu.
>>>
>>>    find_idlest_group also returns NULL when the local group is
>>>    allowed and is the idlest. The caller then continues the
>>>    find_idlest_group search at a lower level of the current CPU's
>>>    sched_domain hierarchy. However new_cpu is not updated. This means
>>>    the search is pointless and we return prev_cpu from
>>>    select_task_rq_fair.
>>>
>>
>> I think its much simpler than that.. but its late, so who knows ;-)
>>
>> Both cases seem predicated on the assumption that we'll return @cpu when
>> we don't find any idler CPU. Consider, if the local group is the idlest,
>> we should stick with @cpu and simply proceed with the child domain.
>>
>> The confusion, and the bugs, seem to have snuck in when we started
>> considering @prev_cpu, whenever that was. The below is mostly code
>> movement to put that whole while(sd) loop into its own function.
>>
>> The effective change is setting @new_cpu = @cpu when we start that loop:
>>
> <snip>
>> ---
>>  kernel/sched/fair.c | 83 +++++++++++++++++++++++++++++++----------------------
>>  1 file changed, 48 insertions(+), 35 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index c77e4b1d51c0..3e77265c480a 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5588,10 +5588,10 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
>>  }
>>
>>  /*
>> - * find_idlest_cpu - find the idlest cpu among the cpus in group.
>> + * find_idlest_group_cpu - find the idlest cpu among the cpus in group.
>>   */
>>  static int
>> -find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>> +find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>>  {
>>         unsigned long load, min_load = ULONG_MAX;
>>         unsigned int min_exit_latency = UINT_MAX;
>> @@ -5640,6 +5640,50 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
>>         return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu;
>>  }
>>
>> +static int
>> +find_idlest_cpu(struct sched_domain *sd, struct task_struct *p, int cpu, int sd_flag)
>> +{
>> +       struct sched_domain *tmp;
>> +       int new_cpu = cpu;
>> +
>> +       while (sd) {
>> +               struct sched_group *group;
>> +               int weight;
>> +
>> +               if (!(sd->flags & sd_flag)) {
>> +                       sd = sd->child;
>> +                       continue;
>> +               }
>> +
>> +               group = find_idlest_group(sd, p, cpu, sd_flag);
>> +               if (!group) {
>> +                       sd = sd->child;
>> +                       continue;
>
> But this will still have the issue of pointlessly searching in the
> local_group when the idlest CPU is in the non-local group? Stemming
> from the fact that find_idlest_group is broken if the local group is
> not allowed.
>
> I believe this is fixed by Brendan's patch? :
>
> "Initializing this_runnable_load and this_avg_load to ULONG_MAX
>    instead of 0. This means in case a1) we now return the idlest
>    non-local group."
>

Yeah, I don't think this is enough, firstly because of affinity and
secondly because I _guess_ we don't want to migrate from prev_cpu to
@cpu if none of the sched_domains have sd_flag set (...right? That would
feel like wake balancing even when there's no SD_BALANCE_WAKE anywhere).

However the code movement helps - I'll combine it with Vincent's
suggestions and post a v2.