Re: [PATCH v2 0/3] reduce quiesce time for lots of name spaces

[Chao Leng <lengchao@huawei.com>]

On 2020/8/7 21:49, Ming Lei wrote:
> On Fri, Aug 07, 2020 at 05:05:59PM +0800, Chao Leng wrote:
>> nvme_stop_queues quiesce queues for all name spaces, now quiesce one by
>> one, if there is lots of name spaces, sync wait long time(more than 10s).
>> Multipath can not fail over to retry quickly, cause io pause long time.
>> This is not expected.
>> To reduce quiesce time, we introduce async mechanism for sync SRCUs
>> and quiesce queue.
>>
> 
> Frankly speaking, I prefer to replace SRCU with percpu_refcount:
> 
> - percpu_refcount has much less memory footprint than SRCU, so we can simply
> move percpu_refcount into request_queue, instead of adding more bytes
> into each hctx by this patch
> 
> - percpu_ref_get()/percpu_ref_put() isn't slower than srcu_read_lock()/srcu_read_unlock().
> 
> - with percpu_refcount, we can remove 'srcu_idx' from hctx_lock/hctx_unlock()
IO pause long time if fail over, this is a serios problem. we need fix
it as soon as possible. SRCU is just used for blocking queue,
non blocking queue need 0 bytes. So more bytes(just 24 bytes) is not
waste.

About using per_cpu to replace SRCU, I suggest separate discussion.
Can you show the patch? This will make it easier to discuss.
> 
> Thanks,
> Ming
> 
> .
>