[libvirt] "[V3] RFC for support cache tune in libvirt"

[Marcelo Tosatti]

On Wed, Jan 11, 2017 at 10:34:00AM -0200, Marcelo Tosatti wrote:
> On Wed, Jan 11, 2017 at 10:19:10AM -0200, Marcelo Tosatti wrote:
> > 
> > Hi,
> > 
> > Comments/questions related to:
> > https://www.redhat.com/archives/libvir-list/2017-January/msg00354.html
> > 
> > 1) root s2600wt:~/linux# virsh cachetune kvm02 --l3.count 2
> > 
> > How does allocation of code/data look like?
> > 
> > 2) 'nodecachestats' command:
> > 
> > 	3. Add new virsh command 'nodecachestats':
> > 	This API is to expose vary cache resouce left on each hardware (cpu
> > 	socket).
> > 	It will be formated as:
> > 	<resource_type>.<resource_id>: left size KiB
> > 
> > Does this take into account that only contiguous regions of cbm masks
> > can be used for allocations?
> > 
> > Also, it should return the amount of free cache on each cacheid.
> > 
> > 3) The interface should support different sizes for different
> > cache-ids. See the KVM-RT use case at 
> > https://www.redhat.com/archives/libvir-list/2017-January/msg00415.html
> > "WHAT THE USER NEEDS TO SPECIFY FOR VIRTUALIZATION (KVM-RT)".
> 
> And when the user specification lacks cacheid of a given socket in
> the system, the code should use the default resctrlfs masks
> (that is for the default group).
> 
> > 4) Usefulness of exposing minimum unit size.
> > 
> > Rather than specify unit sizes (which forces the user 
> > to convert every time the command is executed), why not specify 
> > in kbytes and round up?
> > 
> >       <resctrl name='L3' unit='KiB' cache_size='56320'
> > cache_unit='2816'/>
> > 
> > As noted in item 1 of
> > https://www.redhat.com/archives/libvir-list/2017-January/msg00494.html,
> > "1) Convertion of kbytes (user specification) --> number of CBM bits
> > for host.", 
> > the format where the size is stored is kbytes, so its awkward 
> > to force users and OpenStack to perform the convertion themselves
> > (and zero benefits... nothing changes if you know the unit size).
> 
> 5) Please perform necessary filesystem locking as described
> at Documentation/x86/intel_rdt_ui.txt in the kernel source.

6) libvirt API should expose the cacheid <-> pcpu mapping
(when implementing cacheid support).