Re: [PATCH v10 00/15] Linux RISC-V AIA Support

[Anup Patel]

On Fri, Oct 20, 2023 at 8:10 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
>
> > On Fri, Oct 20, 2023 at 2:17 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >>
> >> Thanks for the quick reply!
> >>
> >> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
> >>
> >> > On Thu, Oct 19, 2023 at 7:13 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >> >>
> >> >> Hi Anup,
> >> >>
> >> >> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
> >> >>
> >> >> > The RISC-V AIA specification is ratified as-per the RISC-V international
> >> >> > process. The latest ratified AIA specifcation can be found at:
> >> >> > https://github.com/riscv/riscv-aia/releases/download/1.0/riscv-interrupts-1.0.pdf
> >> >> >
> >> >> > At a high-level, the AIA specification adds three things:
> >> >> > 1) AIA CSRs
> >> >> >    - Improved local interrupt support
> >> >> > 2) Incoming Message Signaled Interrupt Controller (IMSIC)
> >> >> >    - Per-HART MSI controller
> >> >> >    - Support MSI virtualization
> >> >> >    - Support IPI along with virtualization
> >> >> > 3) Advanced Platform-Level Interrupt Controller (APLIC)
> >> >> >    - Wired interrupt controller
> >> >> >    - In MSI-mode, converts wired interrupt into MSIs (i.e. MSI generator)
> >> >> >    - In Direct-mode, injects external interrupts directly into HARTs
> >> >>
> >> >> Thanks for working on the AIA support! I had a look at the series, and
> >> >> have some concerns about interrupt ID abstraction.
> >> >>
> >> >> A bit of background, for readers not familiar with the AIA details.
> >> >>
> >> >> IMSIC allows for 2047 unique MSI ("msi-irq") sources per hart, and
> >> >> each MSI is dedicated to a certain hart. The series takes the approach
> >> >> to say that there are, e.g., 2047 interrupts ("lnx-irq") globally.
> >> >> Each lnx-irq consists of #harts * msi-irq -- a slice -- and in the
> >> >> slice only *one* msi-irq is acutally used.
> >> >>
> >> >> This scheme makes affinity changes more robust, because the interrupt
> >> >> sources on "other" harts are pre-allocated. On the other hand it
> >> >> requires to propagate irq masking to other harts via IPIs (this is
> >> >> mostly done up setup/tear down). It's also wasteful, because msi-irqs
> >> >> are hogged, and cannot be used.
> >> >>
> >> >> Contemporary storage/networking drivers usually uses queues per core
> >> >> (or a sub-set of cores). The current scheme wastes a lot of msi-irqs.
> >> >> If we instead used a scheme where "msi-irq == lnx-irq", instead of
> >> >> "lnq-irq = {hart 0;msi-irq x , ... hart N;msi-irq x}", there would be
> >> >> a lot MSIs for other users. 1-1 vs 1-N. E.g., if a storage device
> >> >> would like to use 5 queues (5 cores) on a 128 core system, the current
> >> >> scheme would consume 5 * 128 MSIs, instead of just 5.
> >> >>
> >> >> On the plus side:
> >> >> * Changing interrupts affinity will never fail, because the interrupts
> >> >>   on each hart is pre-allocated.
> >> >>
> >> >> On the negative side:
> >> >> * Wasteful interrupt usage, and a system can potientially "run out" of
> >> >>   interrupts. Especially for many core systems.
> >> >> * Interrupt masking need to proagate to harts via IPIs (there's no
> >> >>   broadcast csr in IMSIC), and a more complex locking scheme IMSIC
> >> >>
> >> >> Summary:
> >> >> The current series caps the number of global interrupts to maximum
> >> >> 2047 MSIs for all cores (whole system). A better scheme, IMO, would be
> >> >> to expose 2047 * #harts unique MSIs.
> >> >>
> >> >> I think this could simplify/remove(?) the locking as well.
> >> >
> >> > Exposing 2047 * #harts unique MSIs has multiple issues:
> >> > 1) The irq_set_affinity() does not work for MSIs because each
> >> >      IRQ is not tied to a particular HART. This means we can't
> >> >      balance the IRQ processing load among HARTs.
> >>
> >> Yes, you can balance. In your code, each *active* MSI is still
> >> bound/active to a specific hard together with the affinity mask. In an
> >> 1-1 model you would still need to track the affinity mask, but the
> >> irq_set_affinity() would be different. It would try to allocate a new
> >> MSI from the target CPU, and then switch to having that MSI active.
> >>
> >> That's what x86 does AFAIU, which is also constrained by the # of
> >> available MSIs.
> >>
> >> The downside, as I pointed out, is that the set affinity action can
> >> fail for a certain target CPU.
> >
> > Yes, irq_set_affinity() can fail for the suggested approach plus for
> > RISC-V AIA, one HART does not have access to other HARTs
> > MSI enable/disable bits so the approach will also involve IPI.
>
> Correct, but the current series does a broadcast to all cores, where the
> 1-1 approach is at most an IPI to a single core.
>
> 128+c machines are getting more common, and you have devices that you
> bring up/down on a per-core basis. Broadcasting IPIs to all cores, when
> dealing with a per-core activity is a pretty noisy neighbor.

Broadcast IPI in the current approach is only done upon MSI mask/unmask
operation. It is not done upon set_affinity() of interrupt handling.

>
> This could be fixed in the existing 1-n approach, by not require to sync
> the cores that are not handling the MSI in question. "Lazy disable"

Incorrect. The approach you are suggesting involves an IPI upon every
irq_set_affinity(). This is because a HART can only enable it's own
MSI ID so when an IRQ is moved to from HART A to HART B with
a different ID X on HART B then we will need an IPI in irq_set_affinit()
to enable ID X on HART B.

>
> >> > 2) All wired IRQs for APLIC MSI-mode will also target a
> >> >     fixed HART hence irq_set_affinity() won't work for wired
> >> >     IRQs as well.
> >>
> >> I'm not following here. Why would APLIC put a constraint here? I had a
> >> look at the specs, and I didn't see anything supporting the current
> >> scheme explicitly.
> >
> > Lets say the number of APLIC wired interrupts  are greater than the
> > number of per-CPU IMSIC IDs. In this case, if all wired interrupts are
> > moved to a particular CPU then irq_set_affinity() will fail for some of
> > the wired interrupts.
>
> Right, it's the case of "full remote CPU" again. Thanks for clearing
> that up.
>
> >> > The idea of treating per-HART MSIs as separate IRQs has
> >> > been discussed in the past.
> >>
> >> Aha! I tried to look for it in lore, but didn't find any. Could you
> >> point me to those discussions?
> >
> > This was done 2 years back in the AIA TG meeting when we were
> > doing the PoC for AIA spec.
>
> Ah, too bad. Thanks regardless.
>
> >> My concern is interrupts become a scarce resource with this
> >> implementation, but maybe my view is incorrect. I've seen bare-metal
> >> x86 systems (no VMs) with ~200 cores, and ~2000 interrupts, but maybe
> >> that is considered "a lot of interrupts".
> >>
> >> As long as we don't get into scenarios where we're running out of
> >> interrupts, due to the software design.
> >>
> >
> > The current approach is simpler and ensures irq_set_affinity
> > always works. The limit of max 2047 IDs is sufficient for many
> > systems (if not all).
>
> Let me give you another view. On a 128c system each core has ~16 unique
> interrupts for disposal. E.g. the Intel E800 NIC has more than 2048
> network queue pairs for each PF.

Clearly, this example is a hypothetical and represents a poorly
designed platform.

Having just 16 IDs per-Core is a very poor design choice. In fact, the
Server SoC spec mandates a minimum 255 IDs.

Regarding NICs which support a large number of queues, the driver
will typically enable only one queue per-core and set the affinity to
separate cores. We have user-space data plane applications based
on DPDK which are capable of using a large number of NIC queues
but these applications are polling based and don't use MSIs.

>
> > When we encounter a system requiring a large number of MSIs,
> > we can either:
> > 1) Extend the AIA spec to support greater than 2047 IDs
> > 2) Re-think the approach in the IMSIC driver
> >
> > The choice between #1 and #2 above depends on the
> > guarantees we want for irq_set_affinity().
>
> The irq_set_affinity() behavior is better with this series, but I think
> the other downsides: number of available interrupt sources, and IPI
> broadcast are worse.

The IPI overhead in the approach you are suggesting will be
even bad compared to the IPI overhead of the current approach
because we will end-up doing IPI upon every irq_set_affinity()
in the suggested approach compared to doing IPI upon every
mask/unmask in the current approach.

The biggest advantage of the current approach is a reliable
irq_set_affinity() which is a very valuable thing to have.

ARM systems easily support a large number of LPIs per-core.
For example, GIC-700 supports 56000 LPIs per-core.
(Refer, https://developer.arm.com/documentation/101516/0300/About-the-GIC-700/Features)

In the RISC-V world, we can easily define a small fast track
extension based on S*csrind extension which can allow a
large number of IMSIC IDs per-core.

Instead of addressing problems on a hypothetical system,
I suggest we go ahead with the current approach and deal
with a system having MSI over-subscription when such a
system shows up.

Regards,
Anup