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

[Björn Töpel]

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 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.
  
  
Björn