Re: [RFC PATCH net-next v5 05/11] net/smc: implement DMB-related operations of loopback-ism

[Wen Gu]

On 2024/4/4 19:27, Niklas Schnelle wrote:
> On Thu, 2024-04-04 at 18:20 +0800, Wen Gu wrote:
> > On 2024/4/4 01:20, Gerd Bayer wrote:
> > > On Sun, 2024-03-24 at 21:55 +0800, Wen Gu wrote:
> > >
> > > When I instrumented this to see, why I still see tons of my other
> > > temporary instrumentation messages from the "ism" driver, I found that
> > > in my setup loopback-ism is used rather infrequently.
> > >
> > > I suspect this is due to how the SMC proposals are constructed in
> > > net/smc/af_smc.c and net/smc/smc_pnet.c - and later evaluated in
> > > smc_check_ism_v2_match() - where there is a first-come-first-serve
> > > selection.
> > >
> > > I wonder if one should change that to favour loopback-ism over "real"
> > > ISM devices - and how this could be achieved elegantly.
> > >
> > > Just some food for thought... Probably little you can do on x86.
> >
> > Yes, it is about the priority of available ISM devices, and now it
> > is decided by their order in the smcd_dev_list. The later registered
> > ISMv2 devices(without pnetid) will be added to the beginning of the
> > list (see smcd_register_dev()). So there is a probability that
> > loopback-ism will not be ranked first, since it is added into list
> > earlier during smc_init().
> >
> > If we have the runtime switch of loopback-ism, we can re-active the
> > loopback-ism, that make it be re-added into the beginning of the dev
> > list and be chosen first. Or a new netlink command to adjust the slot
> > order of available ISM devices in the list. As we discussed before,
> > that could be tasks in stage 1 or stage 2.
> >
> > Thanks!
>
> Maybe when adding the ISM devices we could instead make sure that all
> ISM devices are added after loopback and loopback is added in the
> beginning. I think loopback should always be preferred and would
> consider it a bug if it isn't faster too. Between virtio-ism and ISM it
> may be less clear so maybe for stage 2 we would want a priority setting
> and then insert ordered by priority. Thoughts?
I have no objection. If we all agree, I will keep it at the beginning of the list.

Thanks!