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

[Chao Leng <lengchao@huawei.com>]

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.