KVM: halt_polling: provide a way to qualify wakeups during poll

Some wakeups should not be considered a sucessful poll. For example on
s390 I/O interrupts are usually floating, which means that _ALL_ CPUs
would be considered runnable - letting all vCPUs poll all the time for
transactional like workload, even if one vCPU would be enough.
This can result in huge CPU usage for large guests.
This patch lets architectures provide a way to qualify wakeups if they
should be considered a good/bad wakeups in regard to polls.

For s390 the implementation will fence of halt polling for anything but
known good, single vCPU events. The s390 implementation for floating
interrupts does a wakeup for one vCPU, but the interrupt will be delivered
by whatever CPU checks first for a pending interrupt. We prefer the
woken up CPU by marking the poll of this CPU as "good" poll.
This code will also mark several other wakeup reasons like IPI or
expired timers as "good". This will of course also mark some events as
not sucessful. As  KVM on z runs always as a 2nd level hypervisor,
we prefer to not poll, unless we are really sure, though.

This patch successfully limits the CPU usage for cases like uperf 1byte
transactional ping pong workload or wakeup heavy workload like OLTP
while still providing a proper speedup.

This also introduced a new vcpu stat "halt_poll_no_tuning" that marks
wakeups that are considered not good for polling.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Radim Krčmář <rkrcmar@redhat.com> (for an earlier version)
Cc: David Matlack <dmatlack@google.com>
Cc: Wanpeng Li <kernellwp@gmail.com>
[Rename config symbol. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 92a0229044fb..bbcd921d7cb0 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -229,6 +229,7 @@ struct kvm_vcpu {
 	sigset_t sigset;
 	struct kvm_vcpu_stat stat;
 	unsigned int halt_poll_ns;
+	bool valid_wakeup;
 
 #ifdef CONFIG_HAS_IOMEM
 	int mmio_needed;
@@ -1196,4 +1197,18 @@ int kvm_arch_update_irqfd_routing(struct kvm *kvm, unsigned int host_irq,
 				  uint32_t guest_irq, bool set);
 #endif /* CONFIG_HAVE_KVM_IRQ_BYPASS */
 
+#ifdef CONFIG_HAVE_KVM_INVALID_WAKEUPS
+/* If we wakeup during the poll time, was it a sucessful poll? */
+static inline bool vcpu_valid_wakeup(struct kvm_vcpu *vcpu)
+{
+	return vcpu->valid_wakeup;
+}
+
+#else
+static inline bool vcpu_valid_wakeup(struct kvm_vcpu *vcpu)
+{
+	return true;
+}
+#endif /* CONFIG_HAVE_KVM_INVALID_WAKEUPS */
+
 #endif