xen: events: Process event channels notifications in round-robin order.

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

Fixes bug #1115 "Event channel port scanning unfair".

Bugzilla: http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1115
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[ijc: forward ported from linux-2.6.18-xen.hg 324:7fe1c6d02a2b
 various variables have different names in this tree:
  l1  -> pending_words
  l2  -> pending_bits
  l1i -> word_idx
  l2i -> bit_idx]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 65 insertions, 7 deletions

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 6befe6227159..75cc6f50bd29 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -1028,6 +1028,11 @@ irqreturn_t xen_debug_interrupt(int irq, void *dev_id)
 static DEFINE_PER_CPU(unsigned, xed_nesting_count);
 
 /*
+ * Mask out the i least significant bits of w
+ */
+#define MASK_LSBS(w, i) (w & ((~0UL) << i))
+
+/*
  * Search the CPUs pending events bitmasks.  For each one found, map
  * the event number to an irq, and feed it into do_IRQ() for
  * handling.
@@ -1038,6 +1043,9 @@ static DEFINE_PER_CPU(unsigned, xed_nesting_count);
  */
 static void __xen_evtchn_do_upcall(void)
 {
+        static unsigned int last_word_idx = BITS_PER_LONG - 1;
+        static unsigned int last_bit_idx = BITS_PER_LONG - 1;
+        int word_idx, bit_idx;
         int cpu = get_cpu();
         struct shared_info *s = HYPERVISOR_shared_info;
         struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
@@ -1056,17 +1064,50 @@ static void __xen_evtchn_do_upcall(void)
                 wmb();
 #endif
                 pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
+
+                word_idx = last_word_idx;
+                bit_idx = last_bit_idx;
+
                 while (pending_words != 0) {
                         unsigned long pending_bits;
-                        int word_idx = __ffs(pending_words);
-                        pending_words &= ~(1UL << word_idx);
+                        unsigned long words;
+
+                        word_idx = (word_idx + 1) % BITS_PER_LONG;
+                        words = MASK_LSBS(pending_words, word_idx);
+
+                        /*
+                         * If we masked out all events, wrap around to the
+                         * beginning.
+                         */
+                        if (words == 0) {
+                                word_idx = BITS_PER_LONG - 1;
+                                bit_idx = BITS_PER_LONG - 1;
+                                continue;
+                        }
+                        word_idx = __ffs(words);
 
-                        while ((pending_bits = active_evtchns(cpu, s, word_idx)) != 0) {
-                                int bit_idx = __ffs(pending_bits);
-                                int port = (word_idx * BITS_PER_LONG) + bit_idx;
-                                int irq = evtchn_to_irq[port];
+                        do {
+                                unsigned long bits;
+                                int port, irq;
                                 struct irq_desc *desc;
 
+                                pending_bits = active_evtchns(cpu, s, word_idx);
+
+                                bit_idx = (bit_idx + 1) % BITS_PER_LONG;
+                                bits = MASK_LSBS(pending_bits, bit_idx);
+
+                                /* If we masked out all events, move on. */
+                                if (bits == 0) {
+                                        bit_idx = BITS_PER_LONG - 1;
+                                        break;
+                                }
+
+                                bit_idx = __ffs(bits);
+
+                                /* Process port. */
+                                port = (word_idx * BITS_PER_LONG) + bit_idx;
+                                irq = evtchn_to_irq[port];
+
                                 mask_evtchn(port);
                                 clear_evtchn(port);
 
@@ -1075,7 +1116,24 @@ static void __xen_evtchn_do_upcall(void)
                                         if (desc)
                                                 generic_handle_irq_desc(irq, desc);
                                 }
-                        }
+
+                                /*
+                                 * If this is the final port processed, we'll
+                                 * pick up here+1 next time.
+                                 */
+                                last_word_idx = word_idx;
+                                last_bit_idx = bit_idx;
+
+                        } while (bit_idx != BITS_PER_LONG - 1);
+
+                        pending_bits = active_evtchns(cpu, s, word_idx);
+
+                        /*
+                         * We handled all ports, so we can clear the
+                         * selector bit.
+                         */
+                        if (pending_bits == 0)
+                                pending_words &= ~(1UL << word_idx);
                 }
 
                 BUG_ON(!irqs_disabled());