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

[Chao Leng]

On 2020/8/7 17:24, Ming Lei wrote:
> On Fri, Aug 07, 2020 at 05:04:38PM +0800, Chao Leng wrote:
>>
>>
>> On 2020/7/29 12:39, Sagi Grimberg wrote:
>>>
>>>>>>> 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?
>>> .
>> I tried to optimize the patch，support for non blocking queue and
>> blocking queue.
>> See next email.
> 
> Please see the following thread:
> 
> https://lore.kernel.org/linux-block/05f75e89-b6f7-de49-eb9f-a08aa4e0ba4f@kernel.dk/
> 
> Both Keith and Jens didn't think it is a good idea.
If we can support nonblocking queue and blocking queue simplely, this may be a good choice.
Please review the patch first.
> 
> 
> 
> Thanks,
> Ming
> 
> .
>