[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Sagi Grimberg]

>>>> Dynamically allocating each one is possible but not very scalable.
>>>>
>>>> The question is if there is some way, we can do this with on-stack
>>>> or a single on-heap rcu_head or equivalent that can achieve the same
>>>> effect.
>>>
>>> If the hctx structures are guaranteed to stay put, you could count
>>> them and then do a single allocation of an array of rcu_head structures
>>> (or some larger structure containing an rcu_head structure, if needed).
>>> You could then sequence through this array, consuming one rcu_head per
>>> hctx as you processed it.  Once all the callbacks had been invoked,
>>> it would be safe to free the array.
>>>
>>> Sounds too simple, though.  So what am I missing?
>>
>> We don't want higher-order allocations...
> 
> OK, I will bite...  Do multiple lower-order allocations (page size is
> still lower-order, correct?) and link them together.
> 
> Sorry, couldn't resist...

Possible, but I didn't want us to resort to all this complexity and
thought we can find a better, simpler solution.