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Message-ID: <20171115211145.GY21113@char.us.oracle.com>
Date: Wed, 15 Nov 2017 16:11:45 -0500
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: Wanpeng Li <kernellwp@...il.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v5 2/4] KVM: X86: Add paravirt remote TLB flush
On Mon, Nov 13, 2017 at 02:01:18AM -0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> 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@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> 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?
> + }
> +
> if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> apic_set_eoi_write(kvm_guest_apic_eoi_write);
>
> --
> 2.7.4
>
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