[PATCH 09/10] KVM: Implement IPI-aware directed yield candidate selection

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Integrate IPI tracking with directed yield to improve scheduling when
vCPUs spin waiting for IPI responses.

Implement priority-based candidate selection in kvm_vcpu_on_spin()
with three tiers: Priority 1 uses kvm_vcpu_is_ipi_receiver() to
identify confirmed IPI targets within the recency window, addressing
lock holders spinning on IPI acknowledgment. Priority 2 leverages
existing kvm_arch_dy_has_pending_interrupt() for compatibility with
arch-specific fast paths. Priority 3 falls back to conventional
preemption-based logic when yield_to_kernel_mode is requested,
providing a safety net for non-IPI scenarios.

Add kvm_vcpu_is_good_yield_candidate() helper to consolidate these
checks, preventing over-aggressive boosting while enabling targeted
optimization when IPI patterns are detected.

Performance testing (16 pCPUs host, 16 vCPUs/VM):

Dedup (simlarge):
  2 VMs: +47.1% throughput
  3 VMs: +28.1% throughput
  4 VMs:  +1.7% throughput

VIPS (simlarge):
  2 VMs: +26.2% throughput
  3 VMs: +12.7% throughput
  4 VMs:  +6.0% throughput

Gains stem from effective directed yield when vCPUs spin on IPI
delivery, reducing synchronization overhead. The improvement is most
pronounced at moderate overcommit (2-3 VMs) where contention reduction
outweighs context switching cost.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 virt/kvm/kvm_main.c | 52 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 43 insertions(+), 9 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 495e769c7ddf..9cf44b6b396d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3968,6 +3968,47 @@ bool __weak kvm_vcpu_is_ipi_receiver(struct kvm_vcpu *sender, struct kvm_vcpu *r
 	return false;
 }
 
+/*
+ * IPI-aware candidate selection for directed yield
+ *
+ * Priority order:
+ *  1) Confirmed IPI receiver of 'me' within a short window (always boost)
+ *  2) Arch-provided fast pending interrupt (user-mode boost)
+ *  3) Kernel-mode yield: preempted-in-kernel vCPU (traditional boost)
+ *  4) Otherwise, be conservative
+ */
+static bool kvm_vcpu_is_good_yield_candidate(struct kvm_vcpu *me, struct kvm_vcpu *vcpu,
+					     bool yield_to_kernel_mode)
+{
+	/* Priority 1: recently targeted IPI receiver */
+	if (kvm_vcpu_is_ipi_receiver(me, vcpu))
+		return true;
+
+	/* Priority 2: fast pending-interrupt hint (arch-specific). */
+	if (kvm_arch_dy_has_pending_interrupt(vcpu))
+		return true;
+
+	/*
+	 * Minimal preempted gate for remaining cases:
+	 * - If the target is neither a confirmed IPI receiver nor has a fast
+	 *   pending interrupt, require that the target has been preempted.
+	 * - If yielding to kernel mode is requested, additionally require
+	 *   that the target was preempted while in kernel mode.
+	 *
+	 * This avoids expanding the candidate set too aggressively and helps
+	 * prevent overboost in workloads where the IPI context is not
+	 * involved.
+	 */
+	if (!READ_ONCE(vcpu->preempted))
+		return false;
+
+	if (yield_to_kernel_mode &&
+	    !kvm_arch_vcpu_preempted_in_kernel(vcpu))
+		return false;
+
+	return true;
+}
+
 void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 {
 	int nr_vcpus, start, i, idx, yielded;
@@ -4015,15 +4056,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 		if (kvm_vcpu_is_blocking(vcpu) && !vcpu_dy_runnable(vcpu))
 			continue;
 
-		/*
-		 * Treat the target vCPU as being in-kernel if it has a pending
-		 * interrupt, as the vCPU trying to yield may be spinning
-		 * waiting on IPI delivery, i.e. the target vCPU is in-kernel
-		 * for the purposes of directed yield.
-		 */
-		if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
-		    !kvm_arch_dy_has_pending_interrupt(vcpu) &&
-		    !kvm_arch_vcpu_preempted_in_kernel(vcpu))
+		/* IPI-aware candidate selection */
+		if (!kvm_vcpu_is_good_yield_candidate(me, vcpu, yield_to_kernel_mode))
 			continue;
 
 		if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
-- 
2.43.0