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

[Anup Patel <apatel@ventanamicro.com>]

On Fri, Oct 20, 2023 at 10:07 PM Björn Töpel <bjorn@kernel.org> wrote:
>
> Anup Patel <apatel@ventanamicro.com> writes:
>
> > On Fri, Oct 20, 2023 at 8:10 PM Björn Töpel <bjorn@kernel.org> wrote:
> >>
> >> Anup Patel <apatel@ventanamicro.com> writes:
> >>
> >> > On Fri, Oct 20, 2023 at 2:17 PM Björn Töpel <bjorn@kernel.org> wrote:
> >> >>
> >> >> Thanks for the quick reply!
> >> >>
> >> >> Anup Patel <apatel@ventanamicro.com> writes:
> >> >>
> >> >> > On Thu, Oct 19, 2023 at 7:13 PM Björn Töpel <bjorn@kernel.org> wrote:
> >> >> >>
> >> >> >> Hi Anup,
> >> >> >>
> >> >> >> Anup Patel <apatel@ventanamicro.com> 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.
>
> I'm aware. We're on the same page here.
>
> >>
> >> 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.
>
> Yes, the 1-1 approach will require an IPI to one target cpu on affinity
> changes, and similar on mask/unmask.
>
> The 1-n approach, require no-IPI on affinity changes (nice!), but IPI
> broadcast to all cores on mask/unmask (not so nice).
>
> >> >> 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.
>
> You are misreading. A 128c system with 2047 MSIs per-core, will only
> have 16 *per-core unique* (2047/128) interrupts with the current series.
>
> I'm not saying that each IMSIC has 16 IDs, I'm saying that in a 128c
> system with the maximum amount of MSIs possible in the spec, you'll end
> up with 16 *unique* interrupts per core.

-ENOPARSE

I don't see how this applies to the current approach because we treat
MSI ID space as global across cores so if a system has 2047 MSIs
per-core then we have 2047 MSIs across all cores.

>
> > 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.
>
> That's one sample point, and clearly not the only one. There are *many*
> different usage models. Just because you *assign* MSI, doesn't mean they
> are firing all the time.
>
> I can show you a couple of networking setups where this is clearly not
> enough. Each core has a large number of QoS queues, and each queue would
> very much like to have a dedicated MSI.
>
> >> > 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.
>
> Again, very workload dependent.
>
> This series does IPI broadcast on masking/unmasking, which means that
> cores that don't care get interrupted because, say, a network queue-pair
> is setup on another core.
>
> Some workloads never change the irq affinity.

There are various events which irq affinity such as irq balance,
CPU hotplug, system suspend, etc.

Also, the 1-1 approach does IPI upon set_affinity, mask and
unmask whereas the 1-n approach does IPI only upon mask
and unmask.

>
> I'm just pointing out that there are pro/cons with both variants.
>
> > The biggest advantage of the current approach is a reliable
> > irq_set_affinity() which is a very valuable thing to have.
>
> ...and I'm arguing that we're paying a big price for that.
>
> > 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)
>
> Yeah, but this is not the GIC. This is something that looks more like
> the x86 world. We'll be stuck with a lot of implementations with AIA 1.0
> spec, and many cores.

Well, RISC-V AIA is neigher ARM GIG not x86 APIC. All I am saying
is that there are systems with large number per-core interrupt IDs
for handling MSIs.

>
> > 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.
>
> I've pointed out my concerns. We're not agreeing, but hey, I'm just one
> sample point here! I'll leave it here for others to chime in!
>
> Still much appreciate all the hard work on the series!

Thanks, we have disagreements on this topic but this is
certainly a good discussion.

>
>
> Have a nice weekend,

Regards,
Anup

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