On 16-Jul-18 4:00 PM, Eads, Gage wrote:
Hi all,
Does DPDK support forking secondary processes after executing
rte_eal_init()? The l2fwd_fork example and at least one application
(OpenEM: https://sourceforge.net/projects/eventmachine/) use this model,
and they do so by fixing up the EAL internals (e.g. manually changing
process_type from primary to secondary) at the start of the child
process. This feels like a hack, and I can’t find any documentation
describing this model.
Moreover, this approach doesn’t appear to be compatible with recent EAL
changes. For instance, the multi-process communication creates a couple
handler threads (“rte_mp_handle” and “rte_mp_async”) during EAL
initialization. The child processes won’t inherit these threads, and so
won’t be able to participate in multi-process comms. This means the
reworked memory subsystem and upcoming device hotplug support
(http://mails.dpdk.org/archives/dev/2018-July/107704.html) won’t work
with this fork-after-init model.
This is just one example – there may be other features/subsystems that
won’t work. As far as I can tell there is no official stance (though the
l2fwd_fork example implies it’s supported, IMO); I think either DPDK
should either drop the example and not support this model, or support it
and either document its limitations or resolve them. This model could be
an interesting way to run multi-process DPDK on an ASLR-enabled system,
but supporting this wouldn’t be trivial.
Thanks,
Gage
I think it's a very bad idea to use such a model in recent versions of
DPDK. As you have correctly pointed out, IPC will not work in such a
scenario, and given how our memory subsystem relies on IPC, this is a
recipe for memory corruption and divergent memory maps (since
technically both initial and forked processes believe they are primary).
Even hacking rte_config to make DPDK think it's a secondary process will
not work, because the initialization has already completed, but all of
the threads (IPC, interrupt, etc.) are gone and correct IPC socket was
not created, which means the process becomes invisible to the primary
for all intents and purposes.
We _could_ introduce some kind of "official DPDK fork" function that
would fork the process and then restart interrupt, IPC etc. stuff on an
already running instance of DPDK, but that seems like a workaround for a
problem that shouldn't exist in the first place, because such usage is
fundamentally incompatible with DPDK as it stands now.
--
Thanks,
Anatoly
