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

[Ming Lei]

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()

Thanks,
Ming