[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...