[PATCH v3 1/2] blk-mq: add async quiesce interface

[Chao Leng]

On 2020/7/27 11:50, Ming Lei wrote:
> On Mon, Jul 27, 2020 at 11:33:43AM +0800, Chao Leng wrote:
>>
>>
>> On 2020/7/27 10:08, Ming Lei wrote:
>>>> It is at the end and contains exactly what is needed to synchronize. Not
>>> The sync is simply single global synchronize_rcu(), and why bother to add
>>> extra >=40bytes for each hctx.
>>>
>>>> sure what you mean by reuse hctx->srcu?
>>> You already reuses hctx->srcu, but not see reason to add extra rcu_synchronize
>>> to each hctx for just simulating one single synchronize_rcu().
>>
>> To sync srcu together, the extra bytes must be needed, seperate blocking
>> and non blocking queue to two hctx may be a not good choice.
>>
>> There is two choice: the struct rcu_synchronize is added in hctx or in srcu.
>> Though add rcu_synchronize in srcu has a  weakness: the extra bytes is
>> not need if which do not need batch sync srcu, I still think it's better
>> for the SRCU to provide the batch synchronization interface.
> 
> The 'struct rcu_synchronize' can be allocated from heap or even stack(
> if no too many NSs), which is just one shot sync and the API is supposed
> to be called in slow path. No need to allocate it as long lifetime variable.
> Especially 'struct srcu_struct' has already been too fat.

Stack is not suitable, stack can not provide so many space for
many name space. Heap maybe a choice, but need to add abnormal treat
when alloc memory failed, Thus io pause time can not be ensured.
So the extra space may must be needed for batch srcu sync.