[PATCH 2/4] VT-d PI: Randomly Distribute entries to all online pCPUs' pi blocking list

[Chao Gao <chao.gao@intel.com>]

Currently, a blocked vCPU is put in its pCPU's pi blocking list. If
too many vCPUs are blocked on one pCPU, it will incur that the list
grows too long. After a simple analysis, we can have 32k domains and
128 vcpu per domain, thus about 4M vCPUs may be blocked in one pCPU's
PI blocking list. When a wakeup interrupt arrives, the list is
travelled to find some specific vCPUs to wake them up. This travel in
that case would consume much time.

To mitigate this issue, this patch randomly distributes entries to all
online pCPUs' pi blocking list, other than the pCPU which the vCPU
(the entry) is running on. With this method, the number of entries in
the PI blocking list can reduce by N-times theoretically. N is the
number of online pCPUs.

The change in vmx_pi_unblock_vcpu() is for the following case:
vcpu is running -> try to block (this patch changes NSDT to
another random pCPU) but notification comes in time, thus the vcpu
goes back to running station -> VM-entry (we should set NSDT again,
reverting the change we make to NSDT in vmx_vcpu_block())

Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 xen/arch/x86/hvm/vmx/vmx.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index a2dac56..4c9af59 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -103,13 +103,28 @@ void vmx_pi_per_cpu_init(unsigned int cpu)
 static void vmx_vcpu_block(struct vcpu *v)
 {
     unsigned long flags;
-    unsigned int dest;
+    unsigned int dest, dest_cpu;
     spinlock_t *old_lock;
-    spinlock_t *pi_blocking_list_lock =
-		&per_cpu(vmx_pi_blocking, v->processor).lock;
     struct pi_desc *pi_desc = &v->arch.hvm_vmx.pi_desc;
+    spinlock_t *pi_blocking_list_lock;
+
+    /*
+     * After pCPU goes down, the per-cpu PI blocking list is cleared.
+     * To make sure the parameter vCPU is added to the chosen pCPU's
+     * PI blocking list before the list is cleared, just retry when
+     * finding the pCPU has gone down.
+     */
+ retry:
+    dest_cpu = cpumask_any(&cpu_online_map);
+    pi_blocking_list_lock = &per_cpu(vmx_pi_blocking, dest_cpu).lock;
 
     spin_lock_irqsave(pi_blocking_list_lock, flags);
+    if ( unlikely(!cpu_online(dest_cpu)) )
+    {
+        spin_unlock_irqrestore(pi_blocking_list_lock, flags);
+        goto retry;
+    }
+
     old_lock = cmpxchg(&v->arch.hvm_vmx.pi_blocking.lock, NULL,
                        pi_blocking_list_lock);
 
@@ -120,11 +135,11 @@ static void vmx_vcpu_block(struct vcpu *v)
      */
     ASSERT(old_lock == NULL);
 
-    atomic_inc(&per_cpu(vmx_pi_blocking, v->processor).counter);
-    HVMTRACE_4D(VT_D_PI_BLOCK, v->domain->domain_id, v->vcpu_id, v->processor,
-                atomic_read(&per_cpu(vmx_pi_blocking, v->processor).counter));
+    atomic_inc(&per_cpu(vmx_pi_blocking, dest_cpu).counter);
+    HVMTRACE_4D(VT_D_PI_BLOCK, v->domain->domain_id, v->vcpu_id, dest_cpu,
+                atomic_read(&per_cpu(vmx_pi_blocking, dest_cpu).counter));
     list_add_tail(&v->arch.hvm_vmx.pi_blocking.list,
-                  &per_cpu(vmx_pi_blocking, v->processor).list);
+                  &per_cpu(vmx_pi_blocking, dest_cpu).list);
     spin_unlock_irqrestore(pi_blocking_list_lock, flags);
 
     ASSERT(!pi_test_sn(pi_desc));
@@ -134,7 +149,11 @@ static void vmx_vcpu_block(struct vcpu *v)
     ASSERT(pi_desc->ndst ==
            (x2apic_enabled ? dest : MASK_INSR(dest, PI_xAPIC_NDST_MASK)));
 
+    dest = cpu_physical_id(dest_cpu);
+
     write_atomic(&pi_desc->nv, pi_wakeup_vector);
+    write_atomic(&pi_desc->ndst,
+                 (x2apic_enabled ? dest : MASK_INSR(dest, PI_xAPIC_NDST_MASK)));
 }
 
 static void vmx_pi_switch_from(struct vcpu *v)
@@ -163,6 +182,7 @@ static void vmx_pi_unblock_vcpu(struct vcpu *v)
     unsigned long flags;
     spinlock_t *pi_blocking_list_lock;
     struct pi_desc *pi_desc = &v->arch.hvm_vmx.pi_desc;
+    unsigned int dest = cpu_physical_id(v->processor);
 
     /*
      * Set 'NV' field back to posted_intr_vector, so the
@@ -170,6 +190,8 @@ static void vmx_pi_unblock_vcpu(struct vcpu *v)
      * it is running in non-root mode.
      */
     write_atomic(&pi_desc->nv, posted_intr_vector);
+    write_atomic(&pi_desc->ndst,
+                 x2apic_enabled ? dest : MASK_INSR(dest, PI_xAPIC_NDST_MASK));
 
     pi_blocking_list_lock = v->arch.hvm_vmx.pi_blocking.lock;
 
-- 
1.8.3.1


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