Re: [PATCH net-next v3 1/2] net/smc: make wr buffer count configurable

Halil Pasic
2025-09-26T10:13:02Z

On Fri, 26 Sep 2025 10:44:00 +0800
Guangguan Wang <guangguan.wang@linux.alibaba.com> wrote:

> > +
> > +smcr_max_send_wr - INTEGER
> > + So called work request buffers are SMCR link (and RDMA queue pair) level
> > + resources necessary for performing RDMA operations. Since up to 255
> > + connections can share a link group and thus also a link and the number
> > + of the work request buffers is decided when the link is allocated,
> > + depending on the workload it can a bottleneck in a sense that threads
> > + have to wait for work request buffers to become available. Before the
> > + introduction of this control the maximal number of work request buffers
> > + available on the send path used to be hard coded to 16. With this control
> > + it becomes configurable. The acceptable range is between 2 and 2048.
> > +
> > + Please be aware that all the buffers need to be allocated as a physically
> > + continuous array in which each element is a single buffer and has the size
> > + of SMC_WR_BUF_SIZE (48) bytes. If the allocation fails we give up much
> > + like before having this control.
> > +
> > + Default: 16
> > +
> > +smcr_max_recv_wr - INTEGER
> > + So called work request buffers are SMCR link (and RDMA queue pair) level
> > + resources necessary for performing RDMA operations. Since up to 255
> > + connections can share a link group and thus also a link and the number
> > + of the work request buffers is decided when the link is allocated,
> > + depending on the workload it can a bottleneck in a sense that threads
> > + have to wait for work request buffers to become available. Before the
> > + introduction of this control the maximal number of work request buffers
> > + available on the receive path used to be hard coded to 16. With this control
> > + it becomes configurable. The acceptable range is between 2 and 2048.
> > +
> > + Please be aware that all the buffers need to be allocated as a physically
> > + continuous array in which each element is a single buffer and has the size
> > + of SMC_WR_BUF_SIZE (48) bytes. If the allocation fails we give up much
> > + like before having this control.
> > +
> > + Default: 48
>
> Notice that the ratio of smcr_max_recv_wr to smcr_max_send_wr is set to 3:1, with the
> intention of ensuring that the peer QP's smcr_max_recv_wr is three times the local QP's
> smcr_max_send_wr and the local QP's smcr_max_recv_wr is three times the peer QP's
> smcr_max_send_wr, rather than making the local QP's smcr_max_recv_wr three times its own
> smcr_max_send_wr. The purpose of this design is to guarantee sufficient receive WRs on
> the side to receive incoming data when peer QP doing RDMA sends. Otherwise, RNR (Receiver
> Not Ready) may occur, leading to poor performance(RNR will drop the packet and retransmit
> happens in the transport layer of the RDMA).

Thank you Guangguan! I think we already had that discussion.
>
> Let us guess a scenario that have multiple hosts, and the multiple hosts have different
> smcr_max_send_wr and smcr_max_recv_wr configurations, mesh connections between these hosts.
> It is difficult to ensure that the smcr_max_recv_wr/smcr_max_send_wr is 3:1 on the connected
> QPs between these hosts, and it may even be hard to guarantee the smcr_max_recv_wr > smcr_max_send_wr
> on the connected QPs between these hosts.

It is not difficult IMHO. You just leave the knobs alone and you have
3:1 per default. If tuning is attempted that needs to be done carefully.
At least with SMC-R V2 there is this whole EID business, as well so it
is reasonable to assume that the environment can be tuned in a coherent
fashion. E.g. whoever is calling the EID could call use smcr_max_recv_wr:=32 and smcr_max_send_wr:=

>
> Therefore, I believe that if these values are made configurable, additional mechanisms must be
> in place to prevent RNR from occurring. Otherwise we need to carefully configure smcr_max_recv_wr
> and smcr_max_send_wr, or ensure that all hosts capable of establishing SMC-R connections are configured
> smcr_max_recv_wr and smcr_max_send_wr with the same values.

Thank you for