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RE: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: RE: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support
From: "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>
Date: Fri, 11 Dec 2009 09:34:48 +0800
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Cc: Steven Smith <Steven.Smith@xxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxx>
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Thread-topic: [Xen-devel][Pv-ops][PATCH] Netback multiple tasklet support
Jeremy Fitzhardinge wrote:
> On 12/09/09 19:29, Xu, Dongxiao wrote:
>>> Also, is it worth making it a tunable?  Presumably it needn't scale
>>> exactly with the number of dom0 cpus; if you only have one or two
>>> gbit interfaces, then you could saturate that pretty quickly with a
>>> small number of cpus, regardless of how many domains you have.
>> How many CPUs are serving for the NIC interface is determined by how
>> interrupt is delivered. If system only has two gbit interfaces, and
>> they delivier interrupts to CPU0 and CPU1, then the case is: two
>> CPUs handle two tasklets. Other CPUs are idle. The group_nr just
>> defines the max number of tasklets, however it doesn't decide how
>> tasklet is handled by CPU. 
> So does this mean that a given vcpu will be used to handle the
> interrupt if happens to be running on a pcpu with affinity for the
> device?  Or that particular devices will be handled by particular
> vcpus? 

If NIC device is owned by Dom0, then its interrupt affinity is related
with Dom0's *VCPU* (I think its not PCPU). Which VCPU will handle
the device interrupt is determined by the interrupt affinity, either set
manually by commands such as: 
"echo XXX > /proc/irq/irq_num/smp_processor_id",  or automatically 
adjusted by irqbalanced.

>>> I've pushed this out in its own branch:
>>> xen/dom0/backend/netback-tasklet; please post any future patches
>>> against this branch. 
>> What's my next step for this netback-tasklet tree merging into
>> xen/master? 
> Hm, well, I guess:
>     * I'd like to see some comments Keir/Ian(s)/others that this is
>       basically the right approach.  It looks OK to me, but I don't
>       have much experience with performance in the field.
>           o does nc2 make nc1 obsolete?
>     * Testing to make sure it really works.  Netback is clearly
>       critical functionality, so I'd like to be sure we're not
>       introducing big regressions

I will do another round of testing for this patch, and will give you reply 

>      J
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