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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 10, 2020 at 10:17:04AM +0800, Chao Leng wrote:
> 
> 
> 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,

The issue has been long time since SRCU is taken, not sure if it is
something urgent.

> 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.

https://lore.kernel.org/linux-block/20200728134938.1505467-1-ming.lei@redhat.com/


Thanks,
Ming