[Qemu-devel] [PATCH] Tap: fix vcpu long time io blocking on tap

[Wangkai (Kevin,C)]

When used a tap as net driver for vm, if too many packets was delivered to the 
guest os via tap interface, the guest os will be blocked on io events for a long
time, while tap driver was busying process packets.

kvm vcpu thread block on io lock call trace:
  __lll_lock_wait
  _L_lock_1004
  __pthread_mutex_lock
  qemu_mutex_lock
  kvm_cpu_exec
  qemu_kvm_cpu_thread_fn
  start_thread

qemu io thread call trace:
  ...
  qemu_net_queue_send
  tap_send
  qemu_iohandler_poll
  main_loop_wait
  main_loop
  

I think the qemu io lock time should be as small as possible, and the io work
slice should be limited at a particular ration or time.

---
Signed-off-by: Wangkai <wangka...@huawei.com>

diff --git a/net/tap.c b/net/tap.c
index a40f7f0..df9a0eb 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -189,6 +189,7 @@ static void tap_send(void *opaque)
 {
     TAPState *s = opaque;
     int size;
+    int pkt = 0;
 
     while (qemu_can_send_packet(&s->nc)) {
         uint8_t *buf = s->buf;
@@ -210,6 +211,11 @@ static void tap_send(void *opaque)
         } else if (size < 0) {
             break;
         }
+
+        /* limit io block time slice for 50 packets */
+        pkt++;
+        if (pkt >= 50)
+            break;
     }
 }
 
---
2.0.0