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KVM: X86: Bail out of direct yield in case of under-committed scenarios

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In case of under-committed scenarios, vCPUs can be scheduled easily;
kvm_vcpu_yield_to adds extra overhead, and it is also common to see
when vcpu->ready is true but yield later failing due to p->state is
TASK_RUNNING.

Let's bail out in such scenarios by checking the length of current cpu
runqueue, which can be treated as a hint of under-committed instead of
guarantee of accuracy. 30%+ of directed-yield attempts can now avoid
the expensive lookups in kvm_sched_yield() in an under-committed scenario.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1621339235-11131-2-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2021-05-18 05:00:32 -07:00 committed by Paolo Bonzini
parent 6bd5b74368
commit 72b268a8e9
1 changed files with 3 additions and 0 deletions
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arch/x86/kvm/x86.c
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@ -8360,6 +8360,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
vcpu->stat.directed_yield_attempted++;
if (single_task_running())
goto no_yield;
rcu_read_lock();
map = rcu_dereference(vcpu->kvm->arch.apic_map);