Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after_eq().

[annie li <annie.li@xxxxxxxxxx>]

On 2013-10-17 16:26, Jan Beulich wrote:
> > > > On 16.10.13 at 19:22, Jason Luan<jianhai.luan@xxxxxxxxxx>  wrote:
> > time_after_eq() only works if the delta is < MAX_ULONG/2.
> >
> > If netfront sends 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 (e.g., if prior to the long gap, all
> > credit was exhausted).
> >
> > We should add the scenario which now beyond next_credit+MAX_UNLONG/2. Because
> > the fact now must be not before than expire, time_before(now, expire) == true
> > will verify the scenario.
> >      time_after_eq(now, next_credit) || time_before (now, expire)
> >      ==
> >      !time_in_range_open(now, expire, next_credit)
> So first of all this must be with a 32-bit netback. And the not
> coverable gap between activity is well over 240 days long. _If_
> this really needs dealing with, then why is extending this from
> 240+ to 480+ days sufficient?
I am not so sure your mean about extending from 240+ to 480+. Do you 
mean "now" wrapped case happens and falls into the range of from expires 
to next_credit? If this happens, the timer would be set with value based 
on next_credit, which is actually implements the rate control.
Thanks
Annie
> I.e. why don't you simply
> change to 64-bit jiffy values, and use time_after_eq64()?
>
> Jan
>
> > Signed-off-by: Jason Luan<jianhai.luan@xxxxxxxxxx>
> > ---
> >   drivers/net/xen-netback/netback.c |    7 +++++--
> >   1 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/netback.c
> > b/drivers/net/xen-netback/netback.c
> > index f3e591c..31eedaf 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1194,8 +1194,11 @@ static bool tx_credit_exceeded(struct xenvif *vif,
> > unsigned size)
> >         if (timer_pending(&vif->credit_timeout))
> >                 return true;
> >   
> > -	/* Passed the point where we can replenish credit? */
> > -       if (time_after_eq(now, next_credit)) {
> > +       /* Credit should be replenished when now does not fall into the
> > +        * range from expires to next_credit, and time_in_range_open()
> > +        * is used to verify whether this case happens.
> > +        */
> > +       if (!time_in_range_open(now, vif->credit_timeout.expires, next_credit)) 
> > {
> >                 vif->credit_timeout.expires = now;
> >                 tx_add_credit(vif);
> >         }
> > --
> > 1.7.6.5
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx  
> > http://lists.xen.org/xen-devel  

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