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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Thu, 16 Nov 2017 18:17:33 +0800
Message-ID: <CANRm+CziMJWvonGwAhyo8bX_U8iVhWsp5FN2vNk7Q+88TzQjKQ@mail.gmail.com>
Subject: Re: [PATCH v5 2/4] KVM: X86: Add paravirt remote TLB flush
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        kvm <kvm@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Wanpeng Li <wanpeng.li@hotmail.com>
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2017-11-16 5:11 GMT+08:00 Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>:
> On Mon, Nov 13, 2017 at 02:01:18AM -0800, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@hotmail.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@redhat.com>
>> Cc: Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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 featur=
e bit
>> +                                   ||       || before enabling paravirt=
ualized
>> +                                   ||       || tlb flush.
>> +-----------------------------------------------------------------------=
-------
>>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no gue=
st-side
>>                                     ||       || per-cpu warps are expect=
ed in
>>                                     ||       || kvmclock.
>> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uap=
i/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 fiel=
d
>>   * 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 =3D this_cpu_cpumask_var_ptr(__pv_tlb_ma=
sk);
>> +
>> +     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 =3D &per_cpu(steal_time, cpu);
>> +             state =3D 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 =3D 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 =3D 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 the=
re
> 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

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