From: Tetsu Yamamoto <tyamamot@redhat.com>
Date: Tue, 12 Aug 2008 16:07:12 -0400
Subject: [xen] process event channel notifications in round-robin
Message-id: 20080812200712.4832.90895.sendpatchset@pq0-1.lab.bos.redhat.com
O-Subject: [RHEL5.3 PATCH 3/7] xen: Process event channels notifications in round-robin order
Bugzilla: 456171
RH-Acked-by: Chris Lalancette <clalance@redhat.com>
RH-Acked-by: Bill Burns <bburns@redhat.com>

bz456171
# HG changeset patch
# User Ian Campbell <ian.campbell@citrix.com>
# Date 1196433331 0
# Node ID 7fe1c6d02a2bd8d38b6a8b529f166990e4cd1dc3
# Parent  fd879c0688bf123a85dcfa371f863da3542ab8af
Process event channels notifications in round-robin order.

Avoids fairness issue resulting from domain 0 processing lowest
numbered event channel first.

Bugzilla #1115 "Event channel port scanning unfair".

From: Scott Rixner <rixner@rice.edu>
From: Diego Ongaro <dieo.ongaro@rice.edu>
From: Alan L. Cox <alc@rice.edu>
Signed-off-by: Ian Campbell <ian.campbell@xensource.com>

diff --git a/drivers/xen/core/evtchn.c b/drivers/xen/core/evtchn.c
index 96d2c58..c611d94 100644
--- a/drivers/xen/core/evtchn.c
+++ b/drivers/xen/core/evtchn.c
@@ -212,11 +212,14 @@ static DEFINE_PER_CPU(unsigned int, upcall_count) = { 0 };
 /* NB. Interrupts are disabled on entry. */
 asmlinkage void evtchn_do_upcall(struct pt_regs *regs)
 {
-	unsigned long  l1, l2;
-	unsigned int   l1i, l2i, port, count;
-	int            irq, cpu = smp_processor_id();
-	shared_info_t *s = HYPERVISOR_shared_info;
-	vcpu_info_t   *vcpu_info = &s->vcpu_info[cpu];
+	unsigned long       l1, l2;
+	unsigned long       masked_l1, masked_l2;
+	unsigned int        l1i, l2i, port, count;
+	static unsigned int last_processed_l1i = BITS_PER_LONG - 1, last_processed_l2i = BITS_PER_LONG - 1;
+	int                 irq, cpu = smp_processor_id();
+	shared_info_t      *s = HYPERVISOR_shared_info;
+	vcpu_info_t        *vcpu_info = &s->vcpu_info[cpu];
+
 
 	do {
 		/* Avoid a callback storm when we reenable delivery. */
@@ -231,13 +234,36 @@ asmlinkage void evtchn_do_upcall(struct pt_regs *regs)
 		rmb();
 #endif
 		l1 = xchg(&vcpu_info->evtchn_pending_sel, 0);
+
+		l1i = last_processed_l1i;
+		l2i = last_processed_l2i;
+
 		while (l1 != 0) {
-			l1i = __ffs(l1);
-			l1 &= ~(1UL << l1i);
 
-			while ((l2 = active_evtchns(cpu, s, l1i)) != 0) {
-				l2i = __ffs(l2);
+			l1i = (l1i + 1) % BITS_PER_LONG;
+			masked_l1 = l1 & ((~0UL) << l1i);
+
+			if (masked_l1 == 0) { /* if we masked out all events, wrap around to the beginning */
+				l1i = BITS_PER_LONG - 1;
+				l2i = BITS_PER_LONG - 1;
+				continue;
+			}
+			l1i = __ffs(masked_l1);
+
+			do {
+				l2 = active_evtchns(cpu, s, l1i);
+
+				l2i = (l2i + 1) % BITS_PER_LONG;
+				masked_l2 = l2 & ((~0UL) << l2i);
 
+				if (masked_l2 == 0) { /* if we masked out all events, move on */
+					l2i = BITS_PER_LONG - 1;
+					break;
+				}
+
+				l2i = __ffs(masked_l2);
+
+				/* process port */
 				port = (l1i * BITS_PER_LONG) + l2i;
 				if ((irq = evtchn_to_irq[port]) != -1)
 					do_IRQ(irq, regs);
@@ -245,7 +271,17 @@ asmlinkage void evtchn_do_upcall(struct pt_regs *regs)
 					exit_idle();
 					evtchn_device_upcall(port);
 				}
-			}
+
+				/* if this is the final port processed, we'll pick up here+1 next time */
+				last_processed_l1i = l1i;
+				last_processed_l2i = l2i;
+
+			} while (l2i != BITS_PER_LONG - 1);
+
+			l2 = active_evtchns(cpu, s, l1i);
+			if (l2 == 0) /* we handled all ports, so we can clear the selector bit */
+				l1 &= ~(1UL << l1i);
+
 		}
 
 		/* If there were nested callbacks then we have more to do. */