RE: How to rte_epoll_wait for IPC?

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Friday, October 6, 2023 10:53 AM
> To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Harman Kalra
> <hkalra@marvell.com>; Anatoly Burakov <anatoly.burakov@intel.com>; David
> Hunt <david.hunt@intel.com>
> Cc: dev@dpdk.org; nd <nd@arm.com>; nd <nd@arm.com>
> Subject: RE: How to rte_epoll_wait for IPC?
> 
> > From: Honnappa Nagarahalli [mailto:Honnappa.Nagarahalli@arm.com]
> > Sent: Friday, 6 October 2023 17.27
> >
> > > From: Morten Brørup <mb@smartsharesystems.com>
> > > Sent: Friday, October 6, 2023 5:04 AM
> > >
> > > Dear Harman, Anatoly and David,
> > >
> > > I have been looking somewhat into power management, and have a
> > question
> > > about rte_epoll:
> > >
> > > Can I use rte_epoll to wait for an event (or interrupt/signal) in
> > > one
> > EAL thread,
> > > generated by another EAL thread?
> > >
> > > Here's a simple use case with two EAL threads:
> > >
> > > 1. The "ingress" thread receives its packets from the NICs, filters
> > the packets
> > > and puts them into a an rte_ring for the "processing" thread. The
> > "ingress"
> > > thread can sleep and use RX interrupts to wake up, as shown in the
> > l3fwd-
> > > power example. All good.
> > >
> > > 2. The "processing" thread receives its packets from the rte_ring.
> > This thread
> > > should sleep until packets are ready for it in the rte_ring.
> > >
> > > The "ingress" thread knows when it puts packets into the rte_ring,
> > > so
> > it can
> > > signal that event to the "processing" thread, to wake it up; either
> > > as
> > an
> > > interrupt/signal, or through a file descriptor. Is this supported by
> > rte_epoll (or
> > > other DPDK APIs), and how?
> > This feature is supported by using rte_wait_until_equal_xx APIs in
> > rte_ring.
> 
> Thank you for the suggestion, Honnappa.
> 
> rte_wait_until_equal_xx would be an excellent solution for physical appliances.
> 
> However, I am looking for O/S level sleeping, like epoll(), so the hypervisor can
> detect that the thread is idle. (I should have mentioned this!)
WFE is trapped by the hypervisor if there are multiple vCPUs mapped to the same physical core and another vCPU is allowed to run.
If there is only one vCPU mapped, then hypervisor does not trap it. This allows the CPU to go to low power state.

> 
> >
> > >
> > >
> > > In a generic scenario, an EAL thread could be rte_epoll_wait'ing for
> > > a
> > variety of
> > > event sources. This may require a different, more advanced, solution.
> > >
> > > A solution to the simple use case suffices.
> > >
> > >
> > > PS: I'm using the standard acronym IPC in the subject, although I'm
> > asking
> > > about Inter Thread Communication, not Inter Process Communication.
> > > I'm
> > not
> > > looking for a multi process solution.
> > >
> > >
> > > Med venlig hilsen / Kind regards,
> > > -Morten Brørup