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

[annie li <annie.li@xxxxxxxxxx>]

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;
> >       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.
> >         /* 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;

> >           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

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