Re: [Xen-devel] [PATCH net V2] xen-netback: don't move event pointer in TX credit timeout callback

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 15/05/14 17:53, Wei Liu wrote:
> On Thu, May 15, 2014 at 05:34:09PM +0100, Zoltan Kiss wrote:
> [...]
> > > > >                 RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, more_to_do);
> > > > > -               if (!more_to_do)
> > > > > +               if (!more_to_do || rate_limited)
> > > > How about calling timer_pending(&vif->credit_timeout) instead?
> > > timer_pending(&vif->credit_timeout) covers only one of two senarios of
> > > "credit exceeded", see tx_credit_exceeded.
> > The other scenario is when the packet size exceeds the credit. There is no
> > packet here actually, we just want to know if this vif ran out of credit and
> > waiting for the timer to fire.
> >
> > >
> Which place are you referring to? There's packet in the ring, right? So
> you're saying in xenvif_poll "more_to_do" is true and "timer_pending" is
> also true when we come to xenvif_poll again?
The goal of this patch to deschedule NAPI if the vif ran out of credit. 
Either you can carry that information from build_gops via a bool, or you 
can check whether the timer is pending. That's what tx_credit_exceeded 
does as well, and then it checks if the actual packet fits in. But in 
xenvif_poll you don't want to know whether an actual packet fits in, you 
only need the information whether tx_credit_exceeded started the timer 
or not.
If it is, you can be sure there is no more credit. If not, you can keep 
the instance running.
> > > > Also, can this __napi_complete and the callback's napi_schedule race with
> > > > each other? When napi_complete is between removing from the list and
> > > > clearing the bit, and napi_schedule is just test&set the bit, the latter
> > > > won't add the instance to the list again
> > > I think it should be fine. How is it different from what we already have
> > > now? Is this something similar to what David once posted?
> > >
> > >    <1395756505-21573-1-git-send-email-david.vrabel@xxxxxxxxxx>
> > Unfortunately that discussion stalled, and my question were not answered, so
> > I bumped it again. But that's different a bit: it was about racing between
> > the NAPI instance (running in softirq context) and the interrupt. Here the
> > danger is that the NAPI instance and the softirq can race. They both run in
> > softirq context, and even if they were originally on the same CPU, I'm sure
> > if the instance move somewhere else, the timer doesn't follow it.
> This comes back to that original question, doesn't it? That's NAPI
> running on CPU A and raised by CPU B.
>
> Wei.
>
> > Zoli

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