Re: [Xen-devel] netfront/netback multiqueue exhausting grants

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 20/01/16 14:52, Ian Campbell wrote:
> 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?)

All grants refs for Tx and Rx are preallocated (this is the allocation
that is failing above).

David

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