Re: Lockless behavior for CQs in userspace

[Jason Gunthorpe <jgg@mellanox.com>]

On Tue, Mar 17, 2020 at 11:10:15AM -0400, Andrew Boyer wrote:
> 
> > On Mar 17, 2020, at 11:00 AM, Leon Romanovsky <leonro@mellanox.com> wrote:
> > 
> > On Tue, Mar 17, 2020 at 10:45:08AM -0400, Andrew Boyer wrote:
> >> Hello Leon,
> >> I understand that we are not to create new providers that use environment variables to control locking behavior. The ‘new’ way to do it is to use thread domains and parent domains.
> >> 
> >> However, even mlx5 still uses the env var exclusively to control lockless behavior for CQs. Do you have anything in mind for how to extend thread_domains or some other part of the API to cover the CQ case?
> > 
> > Which parameter did you have in mind?
> > I would say that all those parameters are coming from pre-rdma-core era.
> > 
> > Doesn't this commit do what you are asking?
> > https://github.com/linux-rdma/rdma-core/commit/0dbde57c59d2983e848c3dbd9ae93eaf8e7b9405
> > 
> > Thanks
> > 
> >> 
> >> Thank you,
> >> Andrew
> >> 
> 
> You are right - I got thrown off by this:
> 
> > 	if (mlx5_spinlock_init(&cq->lock, !mlx5_single_threaded))
> >                 goto err;
> 
> If IBV_CREATE_CQ_ATTR_SINGLE_THREADED is set, it passes an argument
> to the polling function to skip the lock calls entirely. So it
> doesn’t matter that they are still enabled internally.

Hmm, a thread domain should probably force
IBV_CREATE_CQ_ATTR_SINGLE_THREADED during greation with the new API..

Yishai?

Jason