Re: [PATCH rdma-next 00/10] Enable relaxed ordering for ULPs

[Max Gurtovoy]

On 4/6/2021 2:53 PM, Jason Gunthorpe wrote:
> On Tue, Apr 06, 2021 at 08:09:43AM +0300, Leon Romanovsky wrote:
> > On Tue, Apr 06, 2021 at 10:37:38AM +0800, Honggang LI wrote:
> > > On Mon, Apr 05, 2021 at 08:23:54AM +0300, Leon Romanovsky wrote:
> > > > From: Leon Romanovsky <leonro@xxxxxxxxxx>
> > > >
> > > >  From Avihai,
> > > >
> > > > Relaxed Ordering is a PCIe mechanism that relaxes the strict ordering
> > > > imposed on PCI transactions, and thus, can improve performance.
> > > >
> > > > Until now, relaxed ordering could be set only by user space applications
> > > > for user MRs. The following patch series enables relaxed ordering for the
> > > > kernel ULPs as well. Relaxed ordering is an optional capability, and as
> > > > such, it is ignored by vendors that don't support it.
> > > >
> > > > The following test results show the performance improvement achieved
> > > Did you test this patchset with CPU does not support relaxed ordering?
> > I don't think so, the CPUs that don't support RO are Intel's fourth/fifth-generation
> > and they are not interesting from performance point of view.
> >
> > > We observed significantly performance degradation when run perftest with
> > > relaxed ordering enabled over old CPU.
> > >
> > > https://github.com/linux-rdma/perftest/issues/116
> > The perftest is slightly different, but you pointed to the valid point.
> > We forgot to call pcie_relaxed_ordering_enabled() before setting RO bit
> > and arguably this was needed to be done in perftest too.
> No, the PCI device should not have the RO bit set in this situation.
> It is something mlx5_core needs to do. We can't push this into
> applications.

pcie_relaxed_ordering_enabled is called in 
drivers/net/ethernet/mellanox/mlx5/core/en_common.c so probably need to 
move it to

mlx5_core in this series.



> There should be no performance difference from asking for
> IBV_ACCESS_RELAXED_ORDERING when RO is disabled at the PCI config and
> not asking for it at all.
>
> Either the platform has working relaxed ordering that gives a
> performance gain and the RO config spec bit should be set, or it
> doesn't and the bit should be clear.

is this the case today ?

> This is not something to decide in userspace, or in RDMA. At worst it
> becomes another platform specific PCI tunable people have to set.
>
> I thought the old haswell systems were quirked to disable RO globally
> anyhow?
>
> Jason