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
