Re: [Xen-devel] [PATCH net] xen-netback: use jiffies_64 value to calculate credit timeout

On 2013-10-28 10:36, annie li wrote:

On 2013-10-27 19:11, Wei Liu wrote:

quoted

time_after_eq() only works if the delta is < MAX_ULONG/2.

For a 32bit Dom0, if netfront sends packets at a very low rate, the time
between subsequent calls to tx_credit_exceeded() may exceed MAX_ULONG/2
and the test for timer_after_eq() will be incorrect. Credit will not be
replenished and the guest may become unable to send packets (e.g., if
prior to the long gap, all credit was exhausted).

Use jiffies_64 variant to mitigate this problem for 32bit Dom0.

Suggested-by: Jan Beulich <redacted>
Suggested-by: David Vrabel <redacted>
Signed-off-by: Wei Liu <redacted>
Cc: Ian Campbell <redacted>
Cc: Jason Luan <redacted>
---
  drivers/net/xen-netback/common.h    |    1 +
  drivers/net/xen-netback/interface.c |    4 ++--
  drivers/net/xen-netback/netback.c   |   13 ++++++-------
  3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/net/xen-netback/common.h 

b/drivers/net/xen-netback/common.h
index 5715318..400fea1 100644

--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h

@@ -163,6 +163,7 @@ struct xenvif {
      unsigned long   credit_usec;
      unsigned long   remaining_credit;
      struct timer_list credit_timeout;
+    u64 credit_window_start;
        /* Statistics */
      unsigned long rx_gso_checksum_fixup;

diff --git a/drivers/net/xen-netback/interface.c 

b/drivers/net/xen-netback/interface.c
index 01bb854..8c45b63 100644

--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c

@@ -312,8 +312,8 @@ struct xenvif *xenvif_alloc(struct device 

*parent, domid_t domid,
      vif->credit_bytes = vif->remaining_credit = ~0UL;
      vif->credit_usec  = 0UL;
      init_timer(&vif->credit_timeout);
-    /* Initialize 'expires' now: it's used to track the credit 
window. */
-    vif->credit_timeout.expires = jiffies;
+    /* credit window is tracked in credit_window_start */
+    vif->credit_window_start = get_jiffies_64();
        dev->netdev_ops    = &xenvif_netdev_ops;
      dev->hw_features = NETIF_F_SG | NETIF_F_IP_CSUM | NETIF_F_TSO;

diff --git a/drivers/net/xen-netback/netback.c 

b/drivers/net/xen-netback/netback.c
index f3e591c..1bc0688 100644

--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c

@@ -1185,18 +1185,17 @@ out:
    static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
  {
-    unsigned long now = jiffies;
-    unsigned long next_credit =
-        vif->credit_timeout.expires +
-        msecs_to_jiffies(vif->credit_usec / 1000);
+    u64 now = get_jiffies_64();
+    u64 next_credit = vif->credit_window_start +
+        (u64)msecs_to_jiffies(vif->credit_usec / 1000);

You simply replace "credit_timeout.expires" with 
"vif->credit_window_start" here, and never update 
"vif->credit_window_start" in following code.

quoted

        /* Timer could already be pending in rare cases. */
      if (timer_pending(&vif->credit_timeout))
          return true;
        /* Passed the point where we can replenish credit? */
-    if (time_after_eq(now, next_credit)) {
-        vif->credit_timeout.expires = now;
+    if (time_after_eq64(now, next_credit)) {
+        vif->credit_timeout.expires = (unsigned long)now;

updates credit_window_start as following,
vif->credit_window_start = (unsigned long)now;

both credit_window_start and credit_timeout.expires need to be updated 
here,

vif->credit_window_start = (unsigned long)now;
vif->credit_timeout.expires = (unsigned long)now;

quoted

          tx_add_credit(vif);
      }
  @@ -1207,7 +1206,7 @@ static bool tx_credit_exceeded(struct xenvif 
*vif, unsigned size)
          vif->credit_timeout.function =
              tx_credit_callback;
          mod_timer(&vif->credit_timeout,
-              next_credit);
+              (unsigned long)next_credit);

vif->credit_timeout.expires is unsigned long, and this still causes 
original issue on 32bit system, which works well on 64bit. Or 
rewriting code to avoid uses of vif->credit_timeout.expires, but it is 
complex.

My understanding here is wrong, please ignore this.

Thanks
Annie