xen: events: Make round-robin scan fairer by snapshotting each l2 word once only

(except for starting l2 word, which we scan in two parts).

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
[ijc: forward ported from linux-2.6.18-xen.hg 990:427276ac595d]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 21 insertions, 9 deletions

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index c49cb6d6b838..0c3f17ba3956 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -1045,7 +1045,9 @@ static DEFINE_PER_CPU(unsigned int, current_bit_idx);
  */
 static void __xen_evtchn_do_upcall(void)
 {
+        int start_word_idx, start_bit_idx;
         int word_idx, bit_idx;
+        int i;
         int cpu = get_cpu();
         struct shared_info *s = HYPERVISOR_shared_info;
         struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
@@ -1065,10 +1067,12 @@ static void __xen_evtchn_do_upcall(void)
 #endif
                 pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
 
-                word_idx = __this_cpu_read(current_word_idx);
-                bit_idx = __this_cpu_read(current_bit_idx);
+                start_word_idx = __this_cpu_read(current_word_idx);
+                start_bit_idx = __this_cpu_read(current_bit_idx);
+
+                word_idx = start_word_idx;
 
-                while (pending_words != 0) {
+                for (i = 0; pending_words != 0; i++) {
                         unsigned long pending_bits;
                         unsigned long words;
 
@@ -1084,13 +1088,23 @@ static void __xen_evtchn_do_upcall(void)
                         }
                         word_idx = __ffs(words);
 
+                        pending_bits = active_evtchns(cpu, s, word_idx);
+                        bit_idx = 0; /* usually scan entire word from start */
+                        if (word_idx == start_word_idx) {
+                                /* We scan the starting word in two parts */
+                                if (i == 0)
+                                        /* 1st time: start in the middle */
+                                        bit_idx = start_bit_idx;
+                                else
+                                        /* 2nd time: mask bits done already */
+                                        bit_idx &= (1UL << start_bit_idx) - 1;
+                        }
+
                         do {
                                 unsigned long bits;
                                 int port, irq;
                                 struct irq_desc *desc;
 
-                                pending_bits = active_evtchns(cpu, s, word_idx);
-
                                 bits = MASK_LSBS(pending_bits, bit_idx);
 
                                 /* If we masked out all events, move on. */
@@ -1121,10 +1135,8 @@ static void __xen_evtchn_do_upcall(void)
                                 __this_cpu_write(current_bit_idx, bit_idx);
                         } while (bit_idx != 0);
 
-                        pending_bits = active_evtchns(cpu, s, word_idx);
-
-                        /* If we handled all ports, clear the selector bit. */
-                        if (pending_bits == 0)
+                        /* Scan start_l1i twice; all others once. */
+                        if ((word_idx != start_word_idx) || (i != 0))
                                 pending_words &= ~(1UL << word_idx);
 
                         word_idx = (word_idx + 1) % BITS_PER_LONG;