Re: [RFC PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

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

On Thu, Jul 30, 2020 at 11:18:57AM -0700, Keith Busch wrote:
> On Thu, Jul 30, 2020 at 09:10:48AM -0700, Sagi Grimberg wrote:
> > > > I think it will be a significant improvement to have a single code path.
> > > > The code will be more robust and we won't need to face issues that are
> > > > specific for blocking.
> > > > 
> > > > If the cost is negligible, I think the upside is worth it.
> > > > 
> > > 
> > > rcu_read_lock and rcu_read_unlock has been proved as efficient enough,
> > > and I don't think percpu_refcount is better than it, so I'd suggest to
> > > not switch non-blocking into this way.
> > 
> > It's not a matter of which is better, its a matter of making the code
> > more robust because it has a single code-path. If moving to percpu_ref
> > is negligible, I would suggest to move both, I don't want to have two
> > completely different mechanism for blocking vs. non-blocking.
> 
> FWIW, I proposed an hctx percpu_ref over a year ago (but for a
> completely different reason), and it was measured as too costly.
> 
>   https://lore.kernel.org/linux-block/d4a4b6c0-3ea8-f748-85b0-6b39c5023a6f@kernel.dk/

That is why I don't want to switch non-blocking to percpu-refcount.

However, cost of srcu read lock/unlock is basically similar with percpu
ref.


Thanks,
Ming