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 scorevanilla + 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:
- 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.
- 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?
Paolo
Perhaps you can add a lock event counter to count the number of wait_early events caused by vcpu_is_preempted() being true to see if it really cause a lot more wait_early than without the vcpu_is_preempted() call.
I have no objection to this, I just want to find out the root cause of it.
Cheers, Longman