From: Stefan Fritsch Subject: Re: Mostly-Inverted CPU-Core ordering To: Mark Kettenis Cc: gsgs7878@proton.me, tech@openbsd.org, visa@openbsd.org Date: Wed, 14 Jan 2026 15:47:15 +0100 On Wed, 14 Jan 2026, Mark Kettenis wrote: > > Date: Tue, 13 Jan 2026 18:34:39 +0100 (CET) > > From: Stefan Fritsch > > > > visa@ Theo suggested to cc you because of mips64 > > > > On Tue, 13 Jan 2026, Stefan Fritsch wrote: > > > > > On Mon, 12 Jan 2026, Greg Schaefer wrote: > > > > > > > While debugging another issue, I noticed that amd64/cpu_attach (and others > > > > it appears) reorder the ACPI x2APIC CPU list. In a hybrid world, think that > > > > Intel recommends x2APIC ordering by "CPU power" (P-cores first, E-cores next, > > > > LP-cores last). cpu_info_list contains a static primary (boot-CPU) record > > > > and a linked-list to the remaining application processors. The boot core is > > > > copied to the static allocation, but the remaining cores are head-inserted > > > > (reversed) into the linked-list. > > > > > > > > case CPU_ROLE_AP: > > > > #if defined(MULTIPROCESSOR) > > > > cpu_intr_init(ci); > > > > cpu_start_secondary(ci); > > > > clockqueue_init(&ci->ci_queue); > > > > sched_init_cpu(ci); > > > > ncpus++; > > > > if (ci->ci_flags & CPUF_PRESENT) { > > > > ** ci->ci_next = cpu_info_list->ci_next; > > > > ** cpu_info_list->ci_next = ci; > > > > > > > > Using the example of a hybrid process with 2 P-cores, 2 E-cores and 2 LP-cores, > > > > the resulting ordering is P-Core 0, LP-Core 1, LP-Core 0, E-Core 1, E-core 0, > > > > P-core 0. When NICs setup their interrupt queues (often power-of-two), via > > > > intrmap_create(), they are striped across the cores in that mostly-reversed > > > > order. > > > > > > > > Unclear what, if any, performance impact this has, but it can make debugging > > > > interesting. The simple fix is appending to cpu_info_list instead of reverse > > > > head-inserting. Please ignore if a known/non-issue. > > > > > > I can confirm your analysis. I don't have access to a hybrid CPU system > > > at the moment, but I have Linux cpuinfo and acpi table infos from two > > > Dell laptops with Core i7-1270P and Core i7-1370P and they seem to have > > > the P-cores before the E-cores in the MADT. Also, if I enable INTRDEBUG > > > in amd64/intr.c, I see that CPU_INFO_FOREACH traverses the CPUs in the > > > order 0,5,4,3,2,1 on a 6 VCPU VM and intrmap_create() uses > > > CPU_INFO_FOREACH. > > > > > > I also agree that one probably wants to have multi-queue NICs use the > > > CPUs with the highest performance and re-ordering the cpu_info_list > > > seems to have that effect at least on intel. The amd64 diff below puts > > > them in the correct order and seems to work. But the same code seems to > > > be in most (all?) other MP architectures. > > > > > > I don't know what effect the different ordering could have on non-hybrid > > > systems. On multi-socket systems, the interrupt distribution over the > > > sockets may change, but I don't see a problem with that. The order in > > > which IPIs are send will also change, and the scheduler will likely also > > > do things slightly differently, but I have no idea if that would have a > > > measurable impact. Anything else? > > > > Updated diff below that should fix all architectures. sparc64 already has > > the correct ordering. Boot tested on amd64, i386, arm64. > > On arm64, the slow cores are typically (but not always) listed before > the fast cores. Don't think that is a good reason not to change the > order of the list to be more sensible. Does this mean that the boot cpu is then a slow core, too? In the long term, one probably wants to have some field in cpu_info that denotes the core type and then make intrmap_create() use that info to prefer the fastest cores. As noted by tb@, the previous diff broke !MULTIPROCESSOR configurations. New diff attached. diff --git a/sys/arch/alpha/alpha/cpu.c b/sys/arch/alpha/alpha/cpu.c index 23b25c94219..38425f1d905 100644 --- a/sys/arch/alpha/alpha/cpu.c +++ b/sys/arch/alpha/alpha/cpu.c @@ -419,7 +419,7 @@ cpu_announce_extensions(struct cpu_info *ci) void cpu_boot_secondary_processors(void) { - struct cpu_info *ci; + struct cpu_info *ci, *ci_last; u_long i; for (i = 0; i < ALPHA_MAXPROCS; i++) { @@ -434,8 +434,10 @@ cpu_boot_secondary_processors(void) /* * Link the processor into the list, and launch it. */ - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; atomic_setbits_ulong(&ci->ci_flags, CPUF_RUNNING); atomic_setbits_ulong(&cpus_running, (1UL << i)); ncpus++; diff --git a/sys/arch/amd64/amd64/cpu.c b/sys/arch/amd64/amd64/cpu.c index 18cfb8d6377..4d56f320c33 100644 --- a/sys/arch/amd64/amd64/cpu.c +++ b/sys/arch/amd64/amd64/cpu.c @@ -729,8 +729,11 @@ cpu_attach(struct device *parent, struct device *self, void *aux) sched_init_cpu(ci); ncpus++; if (ci->ci_flags & CPUF_PRESENT) { - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + struct cpu_info *ci_last = cpu_info_list; + + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; } #else printf("%s: not started\n", sc->sc_dev.dv_xname); diff --git a/sys/arch/arm/arm/cpu.c b/sys/arch/arm/arm/cpu.c index f51404f73be..005211ba35f 100644 --- a/sys/arch/arm/arm/cpu.c +++ b/sys/arch/arm/arm/cpu.c @@ -354,10 +354,14 @@ cpu_attach(struct device *parent, struct device *dev, void *aux) ci = &cpu_info_primary; ci->ci_flags |= CPUF_RUNNING | CPUF_PRESENT | CPUF_PRIMARY; } else { + struct cpu_info *ci_last; + ci = malloc(sizeof(*ci), M_DEVBUF, M_WAITOK | M_ZERO); cpu_info[dev->dv_unit] = ci; - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; ci->ci_flags |= CPUF_AP; ncpus++; } diff --git a/sys/arch/arm64/arm64/cpu.c b/sys/arch/arm64/arm64/cpu.c index f9784e432db..8f59fc03e0f 100644 --- a/sys/arch/arm64/arm64/cpu.c +++ b/sys/arch/arm64/arm64/cpu.c @@ -1481,6 +1481,7 @@ cpu_attach(struct device *parent, struct device *dev, void *aux) struct cpu_info *ci; void *kstack; #ifdef MULTIPROCESSOR + struct cpu_info *ci_last; uint64_t mpidr = READ_SPECIALREG(mpidr_el1); #endif uint32_t opp; @@ -1494,8 +1495,10 @@ cpu_attach(struct device *parent, struct device *dev, void *aux) } else { ci = malloc(sizeof(*ci), M_DEVBUF, M_WAITOK | M_ZERO); cpu_info[dev->dv_unit] = ci; - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; ci->ci_flags |= CPUF_AP; ncpus++; } diff --git a/sys/arch/i386/i386/cpu.c b/sys/arch/i386/i386/cpu.c index c5f18dd4433..e3b4d7cc589 100644 --- a/sys/arch/i386/i386/cpu.c +++ b/sys/arch/i386/i386/cpu.c @@ -234,6 +234,7 @@ cpu_attach(struct device *parent, struct device *self, void *aux) struct cpu_info *ci; #ifdef MULTIPROCESSOR + struct cpu_info *ci_last; int cpunum = sc->sc_dev.dv_unit; vaddr_t kstack; struct pcb *pcb; @@ -365,8 +366,10 @@ cpu_attach(struct device *parent, struct device *self, void *aux) identifycpu(ci); clockqueue_init(&ci->ci_queue); sched_init_cpu(ci); - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; ncpus++; #endif break; diff --git a/sys/arch/mips64/mips64/cpu.c b/sys/arch/mips64/mips64/cpu.c index 040669d397b..fe45ce0970c 100644 --- a/sys/arch/mips64/mips64/cpu.c +++ b/sys/arch/mips64/mips64/cpu.c @@ -95,9 +95,13 @@ cpuattach(struct device *parent, struct device *dev, void *aux) panic("unable to allocate cpu_info"); } } else { + struct cpu_info *ci_last; + ci = &cpu_info_secondaries[cpuno - 1]; - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; ci->ci_flags |= CPUF_PRESENT; } #else diff --git a/sys/arch/riscv64/riscv64/cpu.c b/sys/arch/riscv64/riscv64/cpu.c index e8400d4eed3..1c2c2a1341c 100644 --- a/sys/arch/riscv64/riscv64/cpu.c +++ b/sys/arch/riscv64/riscv64/cpu.c @@ -195,10 +195,14 @@ cpu_attach(struct device *parent, struct device *dev, void *aux) ci->ci_flags |= CPUF_RUNNING | CPUF_PRESENT | CPUF_PRIMARY; csr_set(sie, SIE_SSIE); } else { + struct cpu_info *ci_last; + ci = malloc(sizeof(*ci), M_DEVBUF, M_WAITOK | M_ZERO); cpu_info[dev->dv_unit] = ci; - ci->ci_next = cpu_info_list->ci_next; - cpu_info_list->ci_next = ci; + ci_last = cpu_info_list; + while (ci_last->ci_next != NULL) + ci_last = ci_last->ci_next; + ci_last->ci_next = ci; ci->ci_flags |= CPUF_AP; ncpus++; }