Re: [Xen-devel] [PATCH v2 12/16] libxl: get and set soft affinity

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 14/11/13 16:25, Ian Jackson wrote:
> George Dunlap writes ("Re: [PATCH v2 12/16] libxl: get and set soft affinity"):
> > On 14/11/13 15:11, Ian Jackson wrote:
> > > So perhaps we should have a singe function that can change the
> > > cpupool, hard affinity, and soft affinity, all at once ?
> > I think this is probably a good idea.  Would it make sense to basically
> > have libxl_[gs]et_vcpuaffinity2(), which takes a parameter that can
> > specify that the mask is for either hard, soft, or both?
> No, it needs to take at least two separate parameters for the hard and
> soft masks, because the toolstack might want to set the hard affinity
> and soft affinity to different values, without having to think
> carefully about ordering these calls to avoid generating spurious
> warnings and incompatible arrangements.
>
> I think this probably applies to cpupool changes too.  So perhaps the
> function should look a bit like setresuid or something: it should take
> an optional new cpupool (is there a sentinel value that can be used to
> mean "no change"?), an optional new soft mask, and an optional new
> hard mask.
>
> > > What if the application makes a call to change the cpupool, without
> > > touching the affinities ?  Should the affinities be reset
> > > automatically ?
> > I think whatever happens for hard affinities right now should be carried on.
> Maybe it is a bug that it doesn't do anything.  I think it depends how
> we expect people to use this.  If a caller sets the hard affinities
> and then changes the cpupool, are they supposed to always then set the
> hard affinities again to a new suitable value ?
Well in fact, as far as I can tell, it *does* do something.  When moving 
a vcpu to a new pool, it unconditionally calls 
cpumask_setall(v->cpu_affinity) for each vcpu, which will effectively 
erase the hard affinity. (xen/common/schedule.c:sched_move_domain()).
And, when unplugging cpus, if it unplugs the last cpu a vcpu can run on, 
it also resets the affinity to "all".
 -George



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