RE: [PATCH v3 1/4] eal: deprecate old coremask-based EAL parameters

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Friday, 2 May 2025 18.01
> 
> On Fri, May 02, 2025 at 08:51:22AM -0700, Stephen Hemminger wrote:
> > On Fri,  2 May 2025 16:11:31 +0100
> > Bruce Richardson <bruce.richardson@intel.com> wrote:
> >
> > > As the number of cores/cpus on platforms has increased over the
> years,
> > > the use of coremasks rather than core-lists for identifying DPDK
> cores
> > > has become more and more unwieldy. At this point, let's deprecate
> the
> > > coremask-based EAL parameters for future removal, and point users
> to the
> > > core-list based versions instead.
> > >
> > > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> >
> > Agree.
> >
> > There are some more syntax options on Linux kernel command line:
> > https://github.com/torvalds/linux/blob/master/Documentation/admin-
> guide/kernel-parameters.rst#cpu-lists
> >
> > cpu lists
> > Some kernel parameters take a list of CPUs as a value, e.g. isolcpus,
> nohz_full, irqaffinity, rcu_nocbs. The format of this list is:
> >
> > <cpu number>,...,<cpu number>
> > or
> >
> > <cpu number>-<cpu number> (must be a positive range in ascending
> order)
> > or a mixture
> >
> > <cpu number>,...,<cpu number>-<cpu number>
> >
> 
> This we already support.
> 
> > Note that for the special case of a range one can split the range
> into equal sized groups and for each group use some amount from the
> beginning of that group:
> >
> > <cpu number>-<cpu number>:<used size>/<group size>
> > For example one can add to the command line following parameter:
> >
> > isolcpus=1,2,10-20,100-2000:2/25
> > where the final item represents CPUs 100,101,125,126,150,151,...
> >
> > The value "N" can be used to represent the numerically last CPU on
> the system, i.e "foo_cpus=16-N" would be equivalent to "16-31" on a 32
> core system.
> >
> > Keep in mind that "N" is dynamic, so if system changes cause the
> bitmap width to change, such as less cores in the CPU list, then N and
> any ranges using N will also change. Use the same on a small 4 core
> system, and "16-N" becomes "16-3" and now the same boot input will be
> flagged as invalid (start > end).
> >
> > The special case-tolerant group name "all" has a meaning of selecting
> all CPUs, so that "nohz_full=all" is the equivalent of "nohz_full=0-N".
> >
> > The semantics of "N" and "all" is supported on a level of bitmaps and
> holds for all users of bitmap_parselist().
> >
> 
> Those would be good/interesting extensions to have, if someone wants to
> add
> them to EAL. Hopefully after this patchset, adding them should be
> easier as
> I think we should only have one place in EAL where we parse groups like
> that:
> "eal_parse_set()" function.

Yes, such a syntax parser can be added later.

In the Kernel, it's a generic parser for fixed length bit arrays.
I could imagine using it for other purposes than CPUs, e.g. VLANs or ports.

The "N" feature is *must have* in the Kernel, where it's used for Kernel boot parameters, set before any script to determine the number of CPUs on the system can be run.
That's not the case with EAL parameters, where "N" and "all" only would be *nice to have*.