linux-kernel - Re: [PATCH v6 2/4] KVM: X86: Add Paravirt TLB Shootdown

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Date:   Thu, 30 Nov 2017 14:24:36 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 2/4] KVM: X86: Add Paravirt TLB Shootdown

2017-11-30 0:21 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-11-27 20:05-0800, Wanpeng Li:
>> 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.
>>
>> Testing on a Xeon Gold 6142 2.6GHz 2 sockets, 32 cores, 64 threads,
>> so 64 pCPUs, and each VM is 64 vCPUs.
>>
>> ebizzy -M
>>               vanilla    optimized     boost
>> 1VM            46799         48670        4%
>> 2VM            23962         42691       78%
>> 3VM            16152         37539      132%
>>
>> 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>
>> ---
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> @@ -498,6 +498,37 @@ static void __init kvm_apf_trap_init(void)
>>       update_intr_gate(X86_TRAP_PF, async_page_fault);
>>  }
>>
>> +static DEFINE_PER_CPU(cpumask_t, __pv_tlb_mask);
>> +
>> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>> +                     const struct flush_tlb_info *info)
>> +{
>> +     u8 state;
>> +     int cpu;
>> +     struct kvm_steal_time *src;
>> +     cpumask_t *flushmask = &per_cpu(__pv_tlb_mask, smp_processor_id());
>> +
>> +     if (unlikely(!flushmask))
>> +             return;
>
> I don't see how this can be NULL and if it could, we'd have to call
> native_flush_tlb_others() instead of returning anyway.
>
> Also, Peter mentioned that we're wasting memory (default is 1k per CPU)
> when not running on KVM.  Hyper-V hijacks x86_platform.apic_post_init()
> to achieve late allocation.  smp_ops.smp_prepare_cpus seems slightly
> better for our purposes, but I don't really like either.
>
> Couldn't we use use arch_initcall(), or early_initcall() if there are
> complications with allocating after smp_init()?

Do it in v7. In addition, move pv_mmu_ops.flush_tlb_others =
kvm_flush_tlb_others to the arch_initcall() fails to work even if I
disable rodata through grub. So I continue to keep the callback
replacement in kvm_guest_init() and late allocation in
arch_initcall().

Regards,
Wanpeng Li