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

On 2013-10-28 10:58, annie li wrote:

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

On 2013-10-27 19:11, Wei Liu wrote:
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 <jbeulich@xxxxxxxx>
Suggested-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Jason Luan <jianhai.luan@xxxxxxxxxx>
  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;
- /* 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.

        /* 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;

Annie should be correct.
It is good to me but i afraid that get_jiffies_64() and time_after_eq64() will bring some performance's load when transmit is busy in 32bit. I will do some test about the patch.

No matter what, i prefer own patch.


@@ -1207,7 +1206,7 @@ static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
          vif->credit_timeout.function =
-              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.


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