From f7819512996361280b86259222456fcf15aad926 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Wed, 4 Feb 2015 18:20:58 +0100 Subject: [PATCH] kvm: add halt_poll_ns module parameter This patch introduces a new module parameter for the KVM module; when it is present, KVM attempts a bit of polling on every HLT before scheduling itself out via kvm_vcpu_block. This parameter helps a lot for latency-bound workloads---in particular I tested it with O_DSYNC writes with a battery-backed disk in the host. In this case, writes are fast (because the data doesn't have to go all the way to the platters) but they cannot be merged by either the host or the guest. KVM's performance here is usually around 30% of bare metal, or 50% if you use cache=directsync or cache=writethrough (these parameters avoid that the guest sends pointless flush requests, and at the same time they are not slow because of the battery-backed cache). The bad performance happens because on every halt the host CPU decides to halt itself too. When the interrupt comes, the vCPU thread is then migrated to a new physical CPU, and in general the latency is horrible because the vCPU thread has to be scheduled back in. With this patch performance reaches 60-65% of bare metal and, more important, 99% of what you get if you use idle=poll in the guest. This means that the tunable gets rid of this particular bottleneck, and more work can be done to improve performance in the kernel or QEMU. Of course there is some price to pay; every time an otherwise idle vCPUs is interrupted by an interrupt, it will poll unnecessarily and thus impose a little load on the host. The above results were obtained with a mostly random value of the parameter (500000), and the load was around 1.5-2.5% CPU usage on one of the host's core for each idle guest vCPU. The patch also adds a new stat, /sys/kernel/debug/kvm/halt_successful_poll, that can be used to tune the parameter. It counts how many HLT instructions received an interrupt during the polling period; each successful poll avoids that Linux schedules the VCPU thread out and back in, and may also avoid a likely trip to C1 and back for the physical CPU. While the VM is idle, a Linux 4 VCPU VM halts around 10 times per second. Of these halts, almost all are failed polls. During the benchmark, instead, basically all halts end within the polling period, except a more or less constant stream of 50 per second coming from vCPUs that are not running the benchmark. The wasted time is thus very low. Things may be slightly different for Windows VMs, which have a ~10 ms timer tick. The effect is also visible on Marcelo's recently-introduced latency test for the TSC deadline timer. Though of course a non-RT kernel has awful latency bounds, the latency of the timer is around 8000-10000 clock cycles compared to 20000-120000 without setting halt_poll_ns. For the TSC deadline timer, thus, the effect is both a smaller average latency and a smaller variance. Signed-off-by: Paolo Bonzini --- arch/arm/include/asm/kvm_host.h | 1 + arch/arm64/include/asm/kvm_host.h | 1 + arch/mips/include/asm/kvm_host.h | 1 + arch/mips/kvm/mips.c | 1 + arch/powerpc/include/asm/kvm_host.h | 1 + arch/powerpc/kvm/book3s.c | 1 + arch/powerpc/kvm/booke.c | 1 + arch/s390/include/asm/kvm_host.h | 1 + arch/s390/kvm/kvm-s390.c | 1 + arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/x86.c | 1 + include/trace/events/kvm.h | 19 ++++++++++++ virt/kvm/kvm_main.c | 48 ++++++++++++++++++++++++----- 13 files changed, 71 insertions(+), 7 deletions(-) diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h index bde494654bccb6..6a79314bc1dfcf 100644 --- a/arch/arm/include/asm/kvm_host.h +++ b/arch/arm/include/asm/kvm_host.h @@ -148,6 +148,7 @@ struct kvm_vm_stat { }; struct kvm_vcpu_stat { + u32 halt_successful_poll; u32 halt_wakeup; }; diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h index 2c49aa4ac81859..8efde89613f262 100644 --- a/arch/arm64/include/asm/kvm_host.h +++ b/arch/arm64/include/asm/kvm_host.h @@ -165,6 +165,7 @@ struct kvm_vm_stat { }; struct kvm_vcpu_stat { + u32 halt_successful_poll; u32 halt_wakeup; }; diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h index f2c249796ea8ba..ac4fc716062b79 100644 --- a/arch/mips/include/asm/kvm_host.h +++ b/arch/mips/include/asm/kvm_host.h @@ -120,6 +120,7 @@ struct kvm_vcpu_stat { u32 resvd_inst_exits; u32 break_inst_exits; u32 flush_dcache_exits; + u32 halt_successful_poll; u32 halt_wakeup; }; diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c index e97b907840311d..c9eccf5df91203 100644 --- a/arch/mips/kvm/mips.c +++ b/arch/mips/kvm/mips.c @@ -49,6 +49,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "resvd_inst", VCPU_STAT(resvd_inst_exits), KVM_STAT_VCPU }, { "break_inst", VCPU_STAT(break_inst_exits), KVM_STAT_VCPU }, { "flush_dcache", VCPU_STAT(flush_dcache_exits), KVM_STAT_VCPU }, + { "halt_successful_poll", VCPU_STAT(halt_successful_poll), KVM_STAT_VCPU }, { "halt_wakeup", VCPU_STAT(halt_wakeup), KVM_STAT_VCPU }, {NULL} }; diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h index 7efd666a3fa780..8ef05121d3cd90 100644 --- a/arch/powerpc/include/asm/kvm_host.h +++ b/arch/powerpc/include/asm/kvm_host.h @@ -107,6 +107,7 @@ struct kvm_vcpu_stat { u32 emulated_inst_exits; u32 dec_exits; u32 ext_intr_exits; + u32 halt_successful_poll; u32 halt_wakeup; u32 dbell_exits; u32 gdbell_exits; diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c index 888bf466d8c6bb..cfbcdc65420195 100644 --- a/arch/powerpc/kvm/book3s.c +++ b/arch/powerpc/kvm/book3s.c @@ -52,6 +52,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "dec", VCPU_STAT(dec_exits) }, { "ext_intr", VCPU_STAT(ext_intr_exits) }, { "queue_intr", VCPU_STAT(queue_intr) }, + { "halt_successful_poll", VCPU_STAT(halt_successful_poll), }, { "halt_wakeup", VCPU_STAT(halt_wakeup) }, { "pf_storage", VCPU_STAT(pf_storage) }, { "sp_storage", VCPU_STAT(sp_storage) }, diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c index 9b55dec2d6cc11..6c1316a15a2729 100644 --- a/arch/powerpc/kvm/booke.c +++ b/arch/powerpc/kvm/booke.c @@ -62,6 +62,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "inst_emu", VCPU_STAT(emulated_inst_exits) }, { "dec", VCPU_STAT(dec_exits) }, { "ext_intr", VCPU_STAT(ext_intr_exits) }, + { "halt_successful_poll", VCPU_STAT(halt_successful_poll) }, { "halt_wakeup", VCPU_STAT(halt_wakeup) }, { "doorbell", VCPU_STAT(dbell_exits) }, { "guest doorbell", VCPU_STAT(gdbell_exits) }, diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h index d1ecc7fd057904..f79058e3fd98f1 100644 --- a/arch/s390/include/asm/kvm_host.h +++ b/arch/s390/include/asm/kvm_host.h @@ -196,6 +196,7 @@ struct kvm_vcpu_stat { u32 exit_stop_request; u32 exit_validity; u32 exit_instruction; + u32 halt_successful_poll; u32 halt_wakeup; u32 instruction_lctl; u32 instruction_lctlg; diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c index b2371c0fd1f8cb..1dbab2340a6623 100644 --- a/arch/s390/kvm/kvm-s390.c +++ b/arch/s390/kvm/kvm-s390.c @@ -51,6 +51,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "exit_instruction", VCPU_STAT(exit_instruction) }, { "exit_program_interruption", VCPU_STAT(exit_program_interruption) }, { "exit_instr_and_program_int", VCPU_STAT(exit_instr_and_program) }, + { "halt_successful_poll", VCPU_STAT(halt_successful_poll) }, { "halt_wakeup", VCPU_STAT(halt_wakeup) }, { "instruction_lctlg", VCPU_STAT(instruction_lctlg) }, { "instruction_lctl", VCPU_STAT(instruction_lctl) }, diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 848947ac6ade47..a236e39cc385a4 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -655,6 +655,7 @@ struct kvm_vcpu_stat { u32 irq_window_exits; u32 nmi_window_exits; u32 halt_exits; + u32 halt_successful_poll; u32 halt_wakeup; u32 request_irq_exits; u32 irq_exits; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 1373e04e1f19b1..bd7a70be41b35f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -145,6 +145,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "irq_window", VCPU_STAT(irq_window_exits) }, { "nmi_window", VCPU_STAT(nmi_window_exits) }, { "halt_exits", VCPU_STAT(halt_exits) }, + { "halt_successful_poll", VCPU_STAT(halt_successful_poll) }, { "halt_wakeup", VCPU_STAT(halt_wakeup) }, { "hypercalls", VCPU_STAT(hypercalls) }, { "request_irq", VCPU_STAT(request_irq_exits) }, diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h index 6edf1f2028cdb0..6bfe7eec1c2c78 100644 --- a/include/trace/events/kvm.h +++ b/include/trace/events/kvm.h @@ -37,6 +37,25 @@ TRACE_EVENT(kvm_userspace_exit, __entry->errno < 0 ? -__entry->errno : __entry->reason) ); +TRACE_EVENT(kvm_vcpu_wakeup, + TP_PROTO(__u64 ns, bool waited), + TP_ARGS(ns, waited), + + TP_STRUCT__entry( + __field( __u64, ns ) + __field( bool, waited ) + ), + + TP_fast_assign( + __entry->ns = ns; + __entry->waited = waited; + ), + + TP_printk("%s time %lld ns", + __entry->waited ? "wait" : "poll", + __entry->ns) +); + #if defined(CONFIG_HAVE_KVM_IRQFD) TRACE_EVENT(kvm_set_irq, TP_PROTO(unsigned int gsi, int level, int irq_source_id), diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 0c281760a1c5f7..32449e0e9aa8a0 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -66,6 +66,9 @@ MODULE_AUTHOR("Qumranet"); MODULE_LICENSE("GPL"); +unsigned int halt_poll_ns = 0; +module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR); + /* * Ordering of locks: * @@ -1813,29 +1816,60 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn) } EXPORT_SYMBOL_GPL(mark_page_dirty); +static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu) +{ + if (kvm_arch_vcpu_runnable(vcpu)) { + kvm_make_request(KVM_REQ_UNHALT, vcpu); + return -EINTR; + } + if (kvm_cpu_has_pending_timer(vcpu)) + return -EINTR; + if (signal_pending(current)) + return -EINTR; + + return 0; +} + /* * The vCPU has executed a HLT instruction with in-kernel mode enabled. */ void kvm_vcpu_block(struct kvm_vcpu *vcpu) { + ktime_t start, cur; DEFINE_WAIT(wait); + bool waited = false; + + start = cur = ktime_get(); + if (halt_poll_ns) { + ktime_t stop = ktime_add_ns(ktime_get(), halt_poll_ns); + do { + /* + * This sets KVM_REQ_UNHALT if an interrupt + * arrives. + */ + if (kvm_vcpu_check_block(vcpu) < 0) { + ++vcpu->stat.halt_successful_poll; + goto out; + } + cur = ktime_get(); + } while (single_task_running() && ktime_before(cur, stop)); + } for (;;) { prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE); - if (kvm_arch_vcpu_runnable(vcpu)) { - kvm_make_request(KVM_REQ_UNHALT, vcpu); - break; - } - if (kvm_cpu_has_pending_timer(vcpu)) - break; - if (signal_pending(current)) + if (kvm_vcpu_check_block(vcpu) < 0) break; + waited = true; schedule(); } finish_wait(&vcpu->wq, &wait); + cur = ktime_get(); + +out: + trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited); } EXPORT_SYMBOL_GPL(kvm_vcpu_block);