Re: netfront/netback multiqueue exhausting grants

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2016-01-20 at 09:40 -0500, Boris Ostrovsky wrote:
> On 01/20/2016 07:23 AM, Ian Campbell wrote:
> > There have been a few reports recently[0] which relate to a failure of
> > netfront to allocate sufficient grant refs for all the queues:
> > 
> > [    0.533589] xen_netfront: can't alloc rx grant refs
> > [    0.533612] net eth0: only created 31 queues
> > 
> > Which can be worked around by increasing the number of grants on the
> > hypervisor command line or by limiting the number of queues permitted
> > by
> > either back or front using a module param (which was broken but is now
> > fixed on both sides, but I'm not sure it has been backported everywhere
> > such that it is a reliable thing to always tell users as a workaround).
> > 
> > Is there any plan to do anything about the default/out of the box
> > experience? Either limiting the number of queues or making both ends
> > cope
> > more gracefully with failure to create some queues (or both) might be
> > sufficient?
> > 
> > I think the crash after the above in the first link at [0] is fixed? I
> > think that was the purpose of ca88ea1247df "xen-netfront: update
> > num_queues
> > to real created" which was in 4.3.
> 
> I think ca88ea1247df is the solution --- it will limit the number of 
> queues.

That's in 4.4, which the first link at [0] claimed to have tested. I can
see this fixing the crash, but does it really fix the "actually works with
less queues than it tried to get" issue?

In any case having exhausted the grant entries creating queues there aren't
any left to shuffle actual data around, is there? (or are those
preallocated too?)

Ian.

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