On Thu, Sep 07, 2023 at 11:09:26AM +0100, Marc Zyngier wrote: > Xu Zhao recently reported[1] that sending SGIs on large VMs was slower > than expected, specially if targeting vcpus that have a high vcpu > index. They root-caused it to the way we walk the vcpu xarray in the > search of the correct MPIDR, one vcpu at a time, which is of course > grossly inefficient. > > The solution they proposed was, unfortunately, less than ideal, but I > was "nerd snipped" into doing something about it. > > The main idea is to build a small hash table of MPIDR to vcpu > mappings, using the fact that most of the time, the MPIDR values only > use a small number of significant bits and that we can easily compute > a compact index from it. Once we have that, accelerating vcpu lookup > becomes pretty cheap, and we can in turn make SGIs great again. > > It must be noted that since the MPIDR values are controlled by > userspace, it isn't always possible to allocate the hash table > (userspace could build a 32 vcpu VM and allocate one bit of affinity > to each of them, making all the bits significant). We thus always have > an iterative fallback -- if it hurts, don't do that. > > Performance wise, this is very significant: using the KUT micro-bench > test with the following patch (always IPI-ing the last vcpu of the VM) > and running it with large number of vcpus shows a large improvement > (from 3832ns to 2593ns for a 64 vcpu VM, a 32% reduction, measured on > an Ampere Altra). I expect that IPI-happy workloads could benefit from > this. > > Thanks, > > M. > > [1] https://lore.kernel.org/r/20230825015811.5292-1-zhaoxu.35@xxxxxxxxxxxxx > > diff --git a/arm/micro-bench.c b/arm/micro-bench.c > index bfd181dc..f3ac3270 100644 > --- a/arm/micro-bench.c > +++ b/arm/micro-bench.c > @@ -88,7 +88,7 @@ static bool test_init(void) > > irq_ready = false; > gic_enable_defaults(); > - on_cpu_async(1, gic_secondary_entry, NULL); > + on_cpu_async(nr_cpus - 1, gic_secondary_entry, NULL); > > cntfrq = get_cntfrq(); > printf("Timer Frequency %d Hz (Output in microseconds)\n", cntfrq); > @@ -157,7 +157,7 @@ static void ipi_exec(void) > > irq_received = false; > > - gic_ipi_send_single(1, 1); > + gic_ipi_send_single(1, nr_cpus - 1); > > while (!irq_received && tries--) > cpu_relax(); > Got a roughly similar perf improvement (about 28%). Tested-by: Joey Gouly <joey.gouly@xxxxxxx> > > Marc Zyngier (5): > KVM: arm64: Simplify kvm_vcpu_get_mpidr_aff() > KVM: arm64: Build MPIDR to vcpu index cache at runtime > KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is > available > KVM: arm64: vgic-v3: Refactor GICv3 SGI generation > KVM: arm64: vgic-v3: Optimize affinity-based SGI injection > > arch/arm64/include/asm/kvm_emulate.h | 2 +- > arch/arm64/include/asm/kvm_host.h | 28 ++++++ > arch/arm64/kvm/arm.c | 66 +++++++++++++ > arch/arm64/kvm/vgic/vgic-mmio-v3.c | 142 ++++++++++----------------- > 4 files changed, 148 insertions(+), 90 deletions(-)
