Re: [PATCH v2 0/3] allow easier use of high lcore-ids

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Apr 07, 2025 at 12:15:13PM +0200, Morten Brørup wrote:
> > From: Bruce Richardson [mailto:bruce.richardson@intel.com] Sent:
> > Monday, 7 April 2025 11.49
> > 
> > On Mon, Apr 07, 2025 at 09:04:05AM +0200, David Marchand wrote:
> > > Hello Bruce,
> > >
> > > On Tue, Apr 1, 2025 at 4:08 PM Bruce Richardson
> > > <bruce.richardson@intel.com> wrote:
> > > >
> > > > On Mon, Mar 24, 2025 at 05:30:26PM +0000, Bruce Richardson wrote:
> > > > > Traditionally, DPDK has had a direct mapping of internal lcore-
> > ids, to
> > > > > the actual core numbers in use. With higher core count servers
> > becoming
> > > > > more prevalent the issue becomes one of increasing memory
> > footprint when
> > > > > using such a scheme, due to the need to have all arrays
> > dimensioned for
> > > > > all cores on the system, whether or not those cores are in use by
> > the
> > > > > app.
> > > > >
> > > > > Therefore, the decision was made in the past to not expand the
> > > > > build-time RTE_MAX_LCORE value beyond 128. Instead, it was
> > recommended
> > > > > that users use the "--lcores" EAL parameter to take the high-
> > numbered
> > > > > cores they wish to use and map them to lcore-ids within the 0 -
> > 128
> > > > > range. While this works, this is a little clunky as it means that
> > > > > instead of just passing, for example, "-l 130-139", the user must
> > > > > instead pass "--lcores 0@130,1@131,2@132,3@133,...."
> > > > >
> > > > > This patchset attempts to simplify the situation by adding a new
> > flag to
> > > > > do this mapping automatically. To use cores 130-139 and map them
> > to ids
> > > > > 0-9 internally, the EAL args now become: "-l 130-139 --map-lcore-
> > ids",
> > > > > or using the shorter "-M" version of the flag: "-Ml 130-139".
> > > > >
> > > > > Adding this new parameter required some rework of the existing
> > arg
> > > > > parsing code, because in current DPDK the args are parsed and
> > checked in
> > > > > the order they appear on the commandline. This means that using
> > the
> > > > > example above, the core parameter 130-139 will be rejected
> > immediately
> > > > > before the "map-lcore-ids" parameter is seen. To work around
> > this, the
> > > > > core (and service core) parameters are not parsed when seen,
> > instead
> > > > > they are only saved off and parsed after all arguments are
> > parsed. The
> > > > > "-l" and "-c" parameters are converted into "--lcores" arguments,
> > so all
> > > > > assigning of lcore ids is done there in all cases.
> > > > >
> > > > > RFC->v2: * converted printf to DEBUG log * added "-M" as shorter
> > > > > version of flag * added documentation * renamed internal API that
> > > > > was changed to avoid any potential
> > hidden
> > > > >   runtime issues.
> > > > >
> > > > > Bruce Richardson (3): eal: centralize core parameter parsing eal:
> > > > > convert core masks and lists to core sets eal: allow automatic
> > > > > mapping of high lcore ids
> > > > >
> > > > Ping for review.
> > > >
> > > > At a high level, does this feature seem useful to users?
> > >
> > > This seems useful, though I am not I would touch the existing
> > options.
> > > I would have gone with a simple -L option (taking the same kind of
> > > input than -l but with new behavior), and not combine a flag with
> > > existing options.
> > >
> > 
> > That would be an easier patchset to do up. However, it would then mean
> > that we have no less than 4 different ways to specify the cores to use:
> > "- c", "-l", "-L", "--lcores" - and therefore need 4 different sets of
> > parsing options for them, and more checks to ensure we have only one of
> > the 4 specified in any run. That's why I chose the modifier option, and
> > to try and consolidate the core setup a bit.
> > 
> > However, if having a completely new option is preferred, I am happy
> > enough to do up a different patchset for that.
> > 
> > > I scanned through the series, not much to say.  Maybe add a unit test
> > > for new cmdline option.
> > >
> > Sure. Once it's decided what approach (if any) to take, I'll do up a
> > new patchset, taking into account any relevant feedback on this set.
> > 
> > /Bruce
> 
> Changing the EAL parameter parser to a "two pass parser" makes sense.  I
> think checking for existence of more than one lcore specification options
> should suffice; we don't need to accept multiple lcore specification
> options and check for conflicts.
> 
> When remapping, do we need to support gaps in the "lcore" (logical cores)
> array, e.g. for secondary processes, or can it be continuous from 0 to
> the number of specified lcores?
> 
> And are new EAL parameters for this really beneficial?  Doesn't e.g. "-l
> 0-9@130-139,100@140" suffice?
> 
Actually, I believe "0-9@130-139"[1]  is not the same as
"0@130,1@131,2@132,...". The latter affinities one thread to one core ten
times over, while the former affinitizes 10 threads to 10 cores - leaving
those threads free to move about within the 10 cores specified.

Just to confirm, I tweaked our helloworld example to print the cpu affinity
of each core when printing.

./build/examples/dpdk-helloworld --no-pci --lcores '(0-3)@(30-33)'
EAL: Detected CPU lcores: 96
EAL: Detected NUMA nodes: 2
EAL: Detected static linkage of DPDK
EAL: Multi-process socket /run/user/11304126/dpdk/rte/mp_socket
EAL: Selected IOVA mode 'VA'
EAL: VFIO support initialized
hello from core 1, with thread affinity for cores: 30 31 32 33 
hello from core 3, with thread affinity for cores: 30 31 32 33 
hello from core 2, with thread affinity for cores: 30 31 32 33 
hello from core 0, with thread affinity for cores: 30 31 32 33 

./build/examples/dpdk-helloworld --no-pci --lcores '0@30,1@31,2@32,3@33'
EAL: Detected CPU lcores: 96
EAL: Detected NUMA nodes: 2
EAL: Detected static linkage of DPDK
EAL: Multi-process socket /run/user/11304126/dpdk/rte/mp_socket
EAL: Selected IOVA mode 'VA'
EAL: VFIO support initialized
hello from core 1, with thread affinity for cores: 31 hello from core 3, with thread affinity for cores: 
hello from core 2, with thread affinity for cores: 32 
hello from core 0, with thread affinity for cores: 30 
33 

Regards,
/Bruce

[1] This actually needs to be "(0-9)@(130-139)", and with "--lcores", not
just "-l", there are actually different flags with different behaviours