Re: [PATCH v2] blkcg: allocate struct blkcg_gq outside request queue spinlock

[Tahsin Erdogan]

On Thu, Mar 2, 2017 at 11:32 AM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello, Tahsin.
>
> On Wed, Mar 01, 2017 at 03:43:19PM -0800, Tahsin Erdogan wrote:
>> @@ -258,18 +258,22 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>>  struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
>> -                                 struct request_queue *q)
>> +                                 struct request_queue *q, bool wait_ok)
>
> I'm okay with this direction but it probably would be better if the
> parameter is gfp_mask and we branch on __GFP_WAIT in the function.

Agreed, gfp mask is better as a parameter.

>
>>  {
>>       struct blkcg_gq *blkg;
>>
>> @@ -300,7 +304,30 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
>>                       parent = blkcg_parent(parent);
>>               }
>>
>> -             blkg = blkg_create(pos, q, NULL);
>> +             if (wait_ok) {
>> +                     struct blkcg_gq *new_blkg;
>> +
>> +                     spin_unlock_irq(q->queue_lock);
>> +                     rcu_read_unlock();
>> +
>> +                     new_blkg = blkg_alloc(pos, q, GFP_KERNEL);
>> +
>> +                     rcu_read_lock();
>> +                     spin_lock_irq(q->queue_lock);
>> +
>> +                     if (unlikely(!new_blkg))
>> +                             return ERR_PTR(-ENOMEM);
>> +
>> +                     if (unlikely(blk_queue_bypass(q))) {
>> +                             blkg_free(new_blkg);
>> +                             return ERR_PTR(blk_queue_dying(q) ?
>> +                                                     -ENODEV : -EBUSY);
>> +                     }
>> +
>> +                     blkg = blkg_create(pos, q, new_blkg);
>> +             } else
>> +                     blkg = blkg_create(pos, q, NULL);
>
> So, while I'm okay with the approach, now we're creating a hybrid
> approach where we have both pre-allocation and allocation mode
> altering mechanisms.  If we're going to take this route, I think the
> right thing to do is passing down @gfp_mask all the way down to
> blkg_create().
>
>> @@ -789,6 +816,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
>>  {
>>       struct gendisk *disk;
>>       struct blkcg_gq *blkg;
>> +     struct request_queue *q;
>>       struct module *owner;
>>       unsigned int major, minor;
>>       int key_len, part, ret;
>> @@ -812,18 +840,27 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
>>               return -ENODEV;
>>       }
>>
>> +     q = disk->queue;
>> +
>>       rcu_read_lock();
>> -     spin_lock_irq(disk->queue->queue_lock);
>> +     spin_lock_irq(q->queue_lock);
>>
>> -     if (blkcg_policy_enabled(disk->queue, pol))
>> -             blkg = blkg_lookup_create(blkcg, disk->queue);
>> -     else
>> +     if (blkcg_policy_enabled(q, pol)) {
>> +             blkg = blkg_lookup_create(blkcg, q, true /* wait_ok */);
>> +
>> +             /*
>> +              * blkg_lookup_create() may have dropped and reacquired the
>> +              * queue lock. Check policy enabled state again.
>> +              */
>> +             if (!IS_ERR(blkg) && unlikely(!blkcg_policy_enabled(q, pol)))
>> +                     blkg = ERR_PTR(-EOPNOTSUPP);
>
> And let blkg_create() verify these conditions after releasing and
> regrabbing the lock.
>
> This also means that the init path can simply pass in GFP_KERNEL.

I tried that approach, but I encountered two issues that complicate things:

1) Pushing down blk_queue_bypass(q) check in blkg_create() doesn't
quite work because when blkcg_init_queue() calls blkg_create(), the
queue is still in bypassing mode.

2) Pushing down blkcg_policy_enabled() doesn't work well either,
because blkcg_init_queue() doesn't have a policy to pass down. We
could let it pass a NULL parameter but that would make blkg_create
more ugly.