[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...
> 
> So:
> 
>    (1) We don't want to embed the struct in the hctx because we allocate
>    so many of them that this is non-negligable to add for something we
>    typically never use.
> 
>    (2) We don't want to allocate dynamically because it's potentially
>    huge.
> 
> As long as we're using srcu for blocking hctx's, I think it's "pick your
> poison".
> 
> Alternatively, Ming's percpu_ref patch(*) may be worth a look.
> 
>   * https://www.spinics.net/lists/linux-block/msg56976.html1
I'm not opposed to having this. Will require some more testing
as this affects pretty much every driver out there..

If we are going with a lightweight percpu_ref, can we just do
it also for non-blocking hctx and have a single code-path?