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

[Anup Patel]

On Mon, Oct 23, 2023 at 12:32 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> Björn Töpel <bjorn@xxxxxxxxxx> writes:
>
> > Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
> >
> >> On Fri, Oct 20, 2023 at 10:07 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >>>
> >>> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
> >>>
> >>> > 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.
> >>>
> >>> 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.
> >
> > Ok, I'll try again! :-)
> >
> > Let's assume that each core in the 128c system has some per-core
> > resources, say a two NIC queue pairs, and a storage queue pair. This
> > will consume, e.g., 2*2 + 2 (6) MSI sources from the global namespace.
> >
> > If each core does this it'll be 6*128 MSI sources of the global
> > namespace.
> >
> > The maximum number of "privates" MSI sources a core can utilize is 16.
> >
> > I'm trying (it's does seem to go that well ;-)) to point out that it's
> > only 16 unique sources per core. For, say, a 256 core system it would be
> > 8. 2047 MSI sources in a system is not much.
> >
> > Say that I want to spin up 24 NIC queues with one MSI each on each core
> > on my 128c system. That's not possible with this series, while with an
> > 1-1 system it wouldn't be an issue.
> >
> > Clearer, or still weird?
> >
> >>
> >>>
> >>> > 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.
> >
> > An important distinction; When you say IPI on mask/unmask it is a
> > broadcast IPI to *all* cores, which is pretty instrusive.
> >
> > The 1-1 variant does an IPI to a *one* target core.
> >
> >>> 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.
> >
> > Yes, and while that is nice, it's not what IMSIC is.
>
> Some follow-ups, after thinking more about it more over the weekend.
>
> * Do one really need an IPI for irq_set_affinity() for the 1-1 model?
>   Why touch the enable/disable bits when moving interrupts?

In the 1-1 model, the ID on the current HART and target HART upon
irq_set_affinity will be different so we can't leave the unused ID on
current HART enabled because it can lead to spurious interrupts
when the ID on current HART is re-used for some other device.

There is also a possibility of receiving an interrupt while the ID was
moved to a new target HART in-which case we have to detect and
re-trigger interrupt on the new target HART. In fact, x86 APLIC does
an IPI to take care of this case.

>
> * In my book the IMSIC looks very much like the x86 LAPIC, which also
>   has few interrupts (IMSIC <2048, LAPIC 256). The IRQ matrix allocator
>   [1], and a scheme similar to LAPIC [2] would be a good fit. This is
>   the 1-1 model, but more sophisticated than what I've been describing
>   (e.g. properly handling mangaged/regular irqs). As a bonus we would
>   get the IRQ matrix debugfs/tracepoint support.
>

Yes, I have been evaluating the 1-1 model for the past few days. I also
have a working implementation with a simple per-CPU bitmap based
allocator which handles both legacy MSI (block of 1,2,4,8,16, or 32 IDs)
and MSI-X.

The irq matrix allocator needs to be improved for handling legacy MSI
so initially I will post a v11 series which works for me and converging
with irq matrix allocator can be future work.

Regards,
Anup