Re: [PATCH v5 2/4] KVM: X86: Add paravirt remote TLB flush
From: Wanpeng Li
Date: Thu Nov 16 2017 - 05:18:57 EST
2017-11-16 5:11 GMT+08:00 Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>:
> On Mon, Nov 13, 2017 at 02:01:18AM -0800, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> Remote flushing api's does a busy wait which is fine in bare-metal
>> scenario. But with-in the guest, the vcpus might have been pre-empted
>> or blocked. In this scenario, the initator vcpu would end up
>> busy-waiting for a long amount of time.
>>
>> This patch set implements para-virt flush tlbs making sure that it does
>> not wait for vcpus that are sleeping. And all the sleeping vcpus flush
>> the tlb on guest enter.
>>
>> The best result is achieved when we're overcommiting the host by running
>> multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching
>> vCPUs which are not scheduled and avoid the wait on the main CPU.
>>
>> Test on a Haswell i7 desktop 4 cores (2HT), so 8 pCPUs, running ebizzy
>> in one linux guest.
>>
>> ebizzy -M
>> vanilla optimized boost
>> 8 vCPUs 10152 10083 -0.68%
>> 16 vCPUs 1224 4866 297.5%
>> 24 vCPUs 1109 3871 249%
>> 32 vCPUs 1025 3375 229.3%
>>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>> Documentation/virtual/kvm/cpuid.txt | 4 ++++
>> arch/x86/include/uapi/asm/kvm_para.h | 2 ++
>> arch/x86/kernel/kvm.c | 42 +++++++++++++++++++++++++++++++++++-
>> 3 files changed, 47 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
>> index 117066a..9693fcc 100644
>> --- a/Documentation/virtual/kvm/cpuid.txt
>> +++ b/Documentation/virtual/kvm/cpuid.txt
>> @@ -60,6 +60,10 @@ KVM_FEATURE_PV_DEDICATED || 8 || guest checks this feature bit
>> || || mizations such as usage of
>> || || qspinlocks.
>> ------------------------------------------------------------------------------
>> +KVM_FEATURE_PV_TLB_FLUSH || 9 || guest checks this feature bit
>> + || || before enabling paravirtualized
>> + || || tlb flush.
>> +------------------------------------------------------------------------------
>> KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
>> || || per-cpu warps are expected in
>> || || kvmclock.
>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
>> index 6d66556..e267d83 100644
>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>> @@ -26,6 +26,7 @@
>> #define KVM_FEATURE_PV_EOI 6
>> #define KVM_FEATURE_PV_UNHALT 7
>> #define KVM_FEATURE_PV_DEDICATED 8
>> +#define KVM_FEATURE_PV_TLB_FLUSH 9
>>
>> /* The last 8 bits are used to indicate how to interpret the flags field
>> * in pvclock structure. If no bits are set, all flags are ignored.
>> @@ -54,6 +55,7 @@ struct kvm_steal_time {
>>
>> #define KVM_VCPU_NOT_PREEMPTED (0 << 0)
>> #define KVM_VCPU_PREEMPTED (1 << 0)
>> +#define KVM_VCPU_SHOULD_FLUSH (1 << 1)
>>
>> #define KVM_CLOCK_PAIRING_WALLCLOCK 0
>> struct kvm_clock_pairing {
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 66ed3bc..78794c1 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -465,9 +465,40 @@ static void __init kvm_apf_trap_init(void)
>> update_intr_gate(X86_TRAP_PF, async_page_fault);
>> }
>>
>> +static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
>> +
>> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>> + const struct flush_tlb_info *info)
>
> Something is off there..
>> +{
>> + u8 state;
>> + int cpu;
>> + struct kvm_steal_time *src;
>> + struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
>> +
>> + if (unlikely(!flushmask))
>> + return;
>> +
>> + cpumask_copy(flushmask, cpumask);
>> + /*
>> + * We have to call flush only on online vCPUs. And
>> + * queue flush_on_enter for pre-empted vCPUs
>> + */
>> + for_each_cpu(cpu, flushmask) {
>> + src = &per_cpu(steal_time, cpu);
>> + state = READ_ONCE(src->preempted);
>> + if ((state & KVM_VCPU_PREEMPTED)) {
>> + if (try_cmpxchg(&src->preempted, &state,
>> + state | KVM_VCPU_SHOULD_FLUSH))
>> + __cpumask_clear_cpu(cpu, flushmask);
>> + }
>> + }
>> +
>> + native_flush_tlb_others(flushmask, info);
>> +}
>> +
>> void __init kvm_guest_init(void)
>> {
>> - int i;
>> + int i, cpu;
>>
>> if (!kvm_para_available())
>> return;
>> @@ -484,6 +515,15 @@ void __init kvm_guest_init(void)
>> pv_time_ops.steal_clock = kvm_steal_clock;
>> }
>>
>> + if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>> + !kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) {
>> + for_each_possible_cpu(cpu) {
>> + zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
>> + GFP_KERNEL, cpu_to_node(cpu));
>> + }
>> + pv_mmu_ops.flush_tlb_others = kvm_flush_tlb_others;
>
> If we migrate to another host that does not expose this, should the
> flush_tlb_others be reset back to the generic one? Or we don't care that
> much ?
>
> It seems to me that we would end up copying the cpumask, then loop around over
> all the CPUs and then call native_flush_tlb_others again. Not sure if there
> is much of performance problem there, but what is the historical way
> this is handled?
We just enable this feature when !PV_DEDICATED which means vCPUs
overcommit, then the loop is better than waiting the other vCPUs that
are sleeping.
Regards,
Wanpeng Li