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

Keith Busch
2020-07-30T19:27:05Z

On Thu, Jul 30, 2020 at 11:23:58AM -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/
>
> If this is the case, we shouldn't consider this as an alternative at all,
> and move forward with either the original proposal or what
> ming proposed to move a counter to the tagset.

Well, the point I was trying to make is that we shouldn't bother making
blocking and non-blocking dispatchers use the same synchronization since
non-blocking has a very cheap solution that blocking can't use.