Re: [dpdk-dev] [PATCH v3 6/9] eal: register non-EAL threads as lcores

[Thomas Monjalon <thomas@monjalon.net>]

24/06/2020 11:56, Bruce Richardson:
> On Wed, Jun 24, 2020 at 11:23:55AM +0200, David Marchand wrote:
> > On Tue, Jun 23, 2020 at 3:16 PM Ananyev, Konstantin
> > <konstantin.ananyev@intel.com> wrote:
> > > > Even before this series, MP has no protection on lcore placing between
> > > > primary and secondary processes.
> > >
> > > Agree, it is not a new problem, it has been there for a while.
> > > Though making lcore assignment dynamic will make it more noticeable and harder to avoid.
> > > With static only lcore distribution it is much easier to control things.
> > >
> > > > Personally, I have no use for DPDK MP and marking MP as not supporting
> > > > this new feature is tempting for a first phase.
> > > > If this is a strong requirement, I can look at it in a second phase.
> > > > What do you think?
> > >
> > > In theory it is possible to mark this new API as not supported for MP.
> > > At least for now. Though I think it is sort of temporal solution.
> > > AFAIK, MP is used by customers, so sooner or later someone will hit that problem.
> > 
> > I understand this argument.
> > But then we don't see those customers giving feedback.
> > 
> > 
> > > Let say, we do have pdump app/library in our mainline.
> > > As I can see - it will be affected when users will start using this new dynamic lcore API
> > > inside their apps.
> > 
> > Supporting lcore allocation in MP requires exchanges between
> > primary/secondary processes like what we have for memory allocations.
> > It will be quite a beast to get to work fine, while not even knowing
> > if people actually want to use both.
> > 
> > For v4, I added a check to exclude MP and the new API.
> > I am still willing to help if people do care about using both features together.
> 
> I wonder how much we could simplify DPDK generally if we had to enable a
> specific runtime flag to enable multi-process support and it was off by
> default. This would break proc_info I think, but maybe we could provide
> telemetry callbacks to provide the same data, but beyond that it would just
> allow us to know whether a DPDK app is actually using MP, or just running
> as a single process.

Same thought here.
I like the idea of a "mode flag" when multi-process is in use.
Should it be an user explicit flag or an automatic one?