Subject: Re: [PATCH rdma-next v2 2/3] RDMA/rw: Support threshold for registration vs scattering to local pages

From: Leon Romanovsky <[email-masked]>
Date: Mon, 7 Oct 2019 19:03:36 +0300

On Mon, Oct 07, 2019 at 08:07:55AM -0700, Bart Van Assche wrote:
> On 10/7/19 6:59 AM, Leon Romanovsky wrote:
> >   /*
> > - * Check if the device might use memory registration.  This is currently only
> > - * true for iWarp devices. In the future we can hopefully fine tune this based
> > - * on HCA driver input.
> > + * Check if the device might use memory registration. This is currently
> > + * true for iWarp devices and devices that have optimized SGL registration
> > + * logic.
> >    */
>
> The following sentence in the above comment looks confusing to me: "Check if
> the device might use memory registration." That sentence suggests that the
> HCA decides whether or not to use memory registration. Isn't it the RDMA R/W
> code that decides whether or not to use memory registration?

I'm open for any reasonable text, what do you expect to be written there?

>
> > + * For RDMA READs we must use MRs on iWarp and can optionaly use them as an
> > + * optimaztion otherwise.  Additionally we have a debug option to force usage
> > + * of MRs to help testing this code path.
>
> You may want to change "optionaly" into "optionally" and "optimaztion" into
> "optimization".

Thanks

>
> >   static inline bool rdma_rw_io_needs_mr(struct ib_device *dev, u8 port_num,
> >   		enum dma_data_direction dir, int dma_nents)
> >   {
> > -	if (rdma_protocol_iwarp(dev, port_num) && dir == DMA_FROM_DEVICE)
> > -		return true;
> > +	if (dir == DMA_FROM_DEVICE) {
> > +		if (rdma_protocol_iwarp(dev, port_num))
> > +			return true;
> > +		if (dev->attrs.max_sgl_rd && dma_nents > dev->attrs.max_sgl_rd)
> > +			return true;
> > +	}
> >   	if (unlikely(rdma_rw_force_mr))
> >   		return true;
> >   	return false;
>
> Should this function be renamed? The function name suggests if this function
> returns 'true' that using memory registration is mandatory. My understanding
> is if this function returns true for the mlx5 HCA that using memory
> registration improves performance but is not mandatory.

The end result the same, better to work with MR while working with mlx5 for "dma_nents >
dev->attrs.max_sgl_rd",

Thanks

> Thanks,
> Bart.