Re: [Qemu-devel] [RFC 0/6] enable numa configuration before machine_init() from HMP/QMP

[Igor Mammedov]

On Wed, 18 Oct 2017 10:59:11 -0200
Eduardo Habkost <address@hidden> wrote:

> On Tue, Oct 17, 2017 at 06:18:59PM +0200, Igor Mammedov wrote:
> > On Tue, 17 Oct 2017 17:09:26 +0100
> > "Daniel P. Berrange" <address@hidden> wrote:
> >   
> > > On Tue, Oct 17, 2017 at 06:06:35PM +0200, Igor Mammedov wrote:  
> > > > On Tue, 17 Oct 2017 16:07:59 +0100
> > > > "Daniel P. Berrange" <address@hidden> wrote:
> > > >     
> > > > > On Tue, Oct 17, 2017 at 09:27:02AM +0200, Igor Mammedov wrote:    
> > > > > > On Mon, 16 Oct 2017 17:36:36 +0100
> > > > > > "Daniel P. Berrange" <address@hidden> wrote:
> > > > > >       
> > > > > > > On Mon, Oct 16, 2017 at 06:22:50PM +0200, Igor Mammedov wrote:    
> > > > > > >   
> > > > > > > > Series allows to configure NUMA mapping at runtime using QMP/HMP
> > > > > > > > interface. For that to happen it introduces a new '-paused' CLI 
> > > > > > > > option
> > > > > > > > which allows to pause QEMU before machine_init() is run and
> > > > > > > > adds new set-numa-node HMP/QMP commands which in conjuction with
> > > > > > > > info hotpluggable-cpus/query-hotpluggable-cpus allow to 
> > > > > > > > configure
> > > > > > > > NUMA mapping for cpus.        
> > > > > > > 
> > > > > > > What's the problem we're seeking solve here compared to what we 
> > > > > > > currently
> > > > > > > do for NUMA configuration ?      
> > > > > > From RHBZ1382425
> > > > > > "
> > > > > > Current -numa CLI interface is quite limited in terms that allow map
> > > > > > CPUs to NUMA nodes as it requires to provide cpu_index values which 
> > > > > > are non obvious and depend on machine/arch. As result libvirt has to
> > > > > > assume/re-implement cpu_index allocation logic to provide valid 
> > > > > > values for -numa cpus=... QEMU CLI option.      
> > > > > 
> > > > > In broad terms, this problem applies to every device / object libvirt
> > > > > asks QEMU to create. For everything else libvirt is able to assign a
> > > > > "id" string, which is can then use to identify the thing later. The
> > > > > CPU stuff is different because libvirt isn't able to provide 'id'
> > > > > strings for each CPU - QEMU generates a psuedo-id internally which
> > > > > libvirt has to infer. The latter is the same problem we had with
> > > > > devices before '-device' was introduced allowing 'id' naming.
> > > > > 
> > > > > IMHO we should take the same approach with CPUs and start modelling 
> > > > > the individual CPUs as something we can explicitly create with -object
> > > > > or -device. That way libvirt can assign names and does not have to 
> > > > > care about CPU index values, and it all works just the same way as
> > > > > any other devices / object we create
> > > > > 
> > > > > ie instead of:
> > > > > 
> > > > >   -smp 8,sockets=4,cores=2,threads=1
> > > > >   -numa node,nodeid=0,cpus=0-3
> > > > >   -numa node,nodeid=1,cpus=4-7
> > > > > 
> > > > > we could do:
> > > > > 
> > > > >   -object numa-node,id=numa0
> > > > >   -object numa-node,id=numa1
> > > > >   -object cpu,id=cpu0,node=numa0,socket=0,core=0,thread=0
> > > > >   -object cpu,id=cpu1,node=numa0,socket=0,core=1,thread=0
> > > > >   -object cpu,id=cpu2,node=numa0,socket=1,core=0,thread=0
> > > > >   -object cpu,id=cpu3,node=numa0,socket=1,core=1,thread=0
> > > > >   -object cpu,id=cpu4,node=numa1,socket=2,core=0,thread=0
> > > > >   -object cpu,id=cpu5,node=numa1,socket=2,core=1,thread=0
> > > > >   -object cpu,id=cpu6,node=numa1,socket=3,core=0,thread=0
> > > > >   -object cpu,id=cpu7,node=numa1,socket=3,core=1,thread=0    
> > > > the follow up question would be where do "socket=3,core=1,thread=0"
> > > > come from, currently these options are the function of
> > > > (-M foo -smp ...) and can be queried vi query-hotpluggble-cpus at
> > > > runtime after qemu parses -M and -smp options.    
> > >   
> 
> Also, note that in the case of NUMA, having identifiers for CPU
> objects themselves won't be enough. NUMA settings need
> identifiers for CPU slots (even if they are still empty), and
> those slots are provided by the machine, not created by the user.
> 
> 
> > > The sockets/cores/threads topology of CPUs is something that comes from
> > > the libvirt guest XML config  
> > in this case things for libvirt to implement would be to know following 
> > details:
> >    1: which machine/machine version support which set of attributes
> >    2: valid values for these properties depending on machine/machine 
> > version/cpu type  
> 
> The big assumption in this series is that libvirt doesn't know in
> advance how the possible slots for CPUs will look like on each
> machine-type, and need to query them using
> query-hotpluggable-cpus.
yep, that's true and it started with introduction of 'device_add cpu'
where libvirt didn't new what to specify as options for new cpu,
hence query-hotpluggable-cpus were added to provide that information.


> But if this assumption was really true, it would be impossible
> for the user to even decide how the NUMA topology will look like,
> wouldn't it?
> 
> Igor, are you able to give one example of how the user input
> (libvirt XML) for configuring NUMA CPU binding could look like if
> the user didn't know yet what the available sockets/cores/threads
> are?
not sure I parse question but looking at libvirt's domain docs
it mentions
  <numa>
    <cell id='0' cpus='0-3' memory='512000' unit='KiB'/>
    <cell id='1' cpus='4-7' memory='512000' unit='KiB' memAccess='shared'/>
  </numa>

here libvirt assumes that there are cpus with cpu-index in range 0-7
/and probably duplicates logic that calculates cpu-index/
If libvirt would continue to duplicate logic we could skip on
implementing early runtime QMP in QEMU and also drop support for
query-hotpluggable-cpus as libvirt would be able to compute
properties/values on it's own.