Re: [PATCH] Revert "locking/pvqspinlock: Don't wait if vCPU is preempted"

[Wanpeng Li <kernellwp@gmail.com>]

On Mon, 9 Sep 2019 at 19:06, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 09/09/19 12:56, Waiman Long wrote:
> > On 9/9/19 2:40 AM, Wanpeng Li wrote:
> >> From: Wanpeng Li <wanpengli@tencent.com>
> >>
> >> This patch reverts commit 75437bb304b20 (locking/pvqspinlock: Don't wait if
> >> vCPU is preempted), we found great regression caused by this commit.
> >>
> >> Xeon Skylake box, 2 sockets, 40 cores, 80 threads, three VMs, each is 80 vCPUs.
> >> The score of ebizzy -M can reduce from 13000-14000 records/s to 1700-1800
> >> records/s with this commit.
> >>
> >>           Host                       Guest                score
> >>
> >> vanilla + w/o kvm optimizes     vanilla               1700-1800 records/s
> >> vanilla + w/o kvm optimizes     vanilla + revert      13000-14000 records/s
> >> vanilla + w/ kvm optimizes      vanilla               4500-5000 records/s
> >> vanilla + w/ kvm optimizes      vanilla + revert      14000-15500 records/s
> >>
> >> Exit from aggressive wait-early mechanism can result in yield premature and
> >> incur extra scheduling latency in over-subscribe scenario.
> >>
> >> kvm optimizes:
> >> [1] commit d73eb57b80b (KVM: Boost vCPUs that are delivering interrupts)
> >> [2] commit 266e85a5ec9 (KVM: X86: Boost queue head vCPU to mitigate lock waiter preemption)
> >>
> >> Tested-by: loobinliu@tencent.com
> >> Cc: Peter Zijlstra <peterz@infradead.org>
> >> Cc: Thomas Gleixner <tglx@linutronix.de>
> >> Cc: Ingo Molnar <mingo@kernel.org>
> >> Cc: Waiman Long <longman@redhat.com>
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Radim Krčmář <rkrcmar@redhat.com>
> >> Cc: loobinliu@tencent.com
> >> Cc: stable@vger.kernel.org
> >> Fixes: 75437bb304b20 (locking/pvqspinlock: Don't wait if vCPU is preempted)
> >> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> >> ---
> >>  kernel/locking/qspinlock_paravirt.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> >> index 89bab07..e84d21a 100644
> >> --- a/kernel/locking/qspinlock_paravirt.h
> >> +++ b/kernel/locking/qspinlock_paravirt.h
> >> @@ -269,7 +269,7 @@ pv_wait_early(struct pv_node *prev, int loop)
> >>      if ((loop & PV_PREV_CHECK_MASK) != 0)
> >>              return false;
> >>
> >> -    return READ_ONCE(prev->state) != vcpu_running || vcpu_is_preempted(prev->cpu);
> >> +    return READ_ONCE(prev->state) != vcpu_running;
> >>  }
> >>
> >>  /*
> >
> > There are several possibilities for this performance regression:
> >
> > 1) Multiple vcpus calling vcpu_is_preempted() repeatedly may cause some
> > cacheline contention issue depending on how that callback is implemented.
>
> Unlikely, it is a single percpu read.
>
> > 2) KVM may set the preempt flag for a short period whenver an vmexit
> > happens even if a vmenter is executed shortly after. In this case, we
> > may want to use a more durable vcpu suspend flag that indicates the vcpu
> > won't get a real vcpu back for a longer period of time.
>
> It sets it for exits to userspace, but they shouldn't really happen on a
> properly-configured system.
>
> However, it's easy to test this theory:
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2e302e977dac..feb6c75a7a88 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3368,26 +3368,28 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
>         int idx;
>
> -       if (vcpu->preempted)
> +       if (vcpu->preempted) {
>                 vcpu->arch.preempted_in_kernel = !kvm_x86_ops->get_cpl(vcpu);
>
> -       /*
> -        * Disable page faults because we're in atomic context here.
> -        * kvm_write_guest_offset_cached() would call might_fault()
> -        * that relies on pagefault_disable() to tell if there's a
> -        * bug. NOTE: the write to guest memory may not go through if
> -        * during postcopy live migration or if there's heavy guest
> -        * paging.
> -        */
> -       pagefault_disable();
> -       /*
> -        * kvm_memslots() will be called by
> -        * kvm_write_guest_offset_cached() so take the srcu lock.
> -        */
> -       idx = srcu_read_lock(&vcpu->kvm->srcu);
> -       kvm_steal_time_set_preempted(vcpu);
> -       srcu_read_unlock(&vcpu->kvm->srcu, idx);
> -       pagefault_enable();
> +               /*
> +                * Disable page faults because we're in atomic context here.
> +                * kvm_write_guest_offset_cached() would call might_fault()
> +                * that relies on pagefault_disable() to tell if there's a
> +                * bug. NOTE: the write to guest memory may not go through if
> +                * during postcopy live migration or if there's heavy guest
> +                * paging.
> +                */
> +               pagefault_disable();
> +               /*
> +                * kvm_memslots() will be called by
> +                * kvm_write_guest_offset_cached() so take the srcu lock.
> +                */
> +               idx = srcu_read_lock(&vcpu->kvm->srcu);
> +               kvm_steal_time_set_preempted(vcpu);
> +               srcu_read_unlock(&vcpu->kvm->srcu, idx);
> +               pagefault_enable();
> +       }
> +
>         kvm_x86_ops->vcpu_put(vcpu);
>         vcpu->arch.last_host_tsc = rdtsc();
>         /*
>
> Wanpeng, can you try?

Yes, there is no difference for the score.

Wanpeng