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

Daniel P. Berrange berrange at redhat.com
Wed Jan 11 10:05:26 UTC 2017 

On Tue, Jan 10, 2017 at 07:42:59AM +0000, Qiao, Liyong wrote:
> Add support for cache allocation.
> 
> Thanks Martin for the previous version comments, this is the v3 version for RFC , I’v have some PoC code [2]. The follow changes are partly finished by the PoC.
> 
> #Propose Changes
> 
> ## virsh command line
> 
> 1. Extend output of nodeinfo, to expose L3 cache size for Level 3 (last level cache size).
> 
> This will expose how many cache on a host which can be used.
> 
> root at s2600wt:~/linux# virsh nodeinfo | grep L3
> L3 cache size:       56320 KiB

Ok, as previously discussed, we should include this in the capabilities
XML instead and have info about all the caches. We likely also want to
relate which CPUs are associated with which cache in some way.

eg if we have this topology

    <topology>
      <cells num='2'>
        <cell id='0'>
          <cpus num='6'>
            <cpu id='0' socket_id='0' core_id='0' siblings='0'/>
            <cpu id='1' socket_id='0' core_id='2' siblings='1'/>
            <cpu id='2' socket_id='0' core_id='4' siblings='2'/>
            <cpu id='6' socket_id='0' core_id='1' siblings='6'/>
            <cpu id='7' socket_id='0' core_id='3' siblings='7'/>
            <cpu id='8' socket_id='0' core_id='5' siblings='8'/>
          </cpus>
        </cell>
        <cell id='1'>
          <cpus num='6'>
            <cpu id='3' socket_id='1' core_id='0' siblings='3'/>
            <cpu id='4' socket_id='1' core_id='2' siblings='4'/>
            <cpu id='5' socket_id='1' core_id='4' siblings='5'/>
            <cpu id='9' socket_id='1' core_id='1' siblings='9'/>
            <cpu id='10' socket_id='1' core_id='3' siblings='10'/>
            <cpu id='11' socket_id='1' core_id='5' siblings='11'/>
          </cpus>
        </cell>
      </cells>
    </topology>

We might have something like this cache info

    <cache>
      <bank type="l3" size="56320" units="KiB" cpus="0,2,3,6,7,8"/>
      <bank type="l3" size="56320" units="KiB" cpus="3,4,5,9,10,11"/>
      <bank type="l2" size="256" units="KiB" cpus="0"/>
      <bank type="l2" size="256" units="KiB" cpus="1"/>
      <bank type="l2" size="256" units="KiB" cpus="2"/>
      <bank type="l2" size="256" units="KiB" cpus="3"/>
      <bank type="l2" size="256" units="KiB" cpus="4"/>
      <bank type="l2" size="256" units="KiB" cpus="5"/>
      <bank type="l2" size="256" units="KiB" cpus="6"/>
      <bank type="l2" size="256" units="KiB" cpus="7"/>
      <bank type="l2" size="256" units="KiB" cpus="8"/>
      <bank type="l2" size="256" units="KiB" cpus="9"/>
      <bank type="l2" size="256" units="KiB" cpus="10"/>
      <bank type="l2" size="256" units="KiB" cpus="11"/>
      <bank type="l1i" size="256" units="KiB" cpus="0"/>
      <bank type="l1i" size="256" units="KiB" cpus="1"/>
      <bank type="l1i" size="256" units="KiB" cpus="2"/>
      <bank type="l1i" size="256" units="KiB" cpus="3"/>
      <bank type="l1i" size="256" units="KiB" cpus="4"/>
      <bank type="l1i" size="256" units="KiB" cpus="5"/>
      <bank type="l1i" size="256" units="KiB" cpus="6"/>
      <bank type="l1i" size="256" units="KiB" cpus="7"/>
      <bank type="l1i" size="256" units="KiB" cpus="8"/>
      <bank type="l1i" size="256" units="KiB" cpus="9"/>
      <bank type="l1i" size="256" units="KiB" cpus="10"/>
      <bank type="l1i" size="256" units="KiB" cpus="11"/>
      <bank type="l1d" size="256" units="KiB" cpus="0"/>
      <bank type="l1d" size="256" units="KiB" cpus="1"/>
      <bank type="l1d" size="256" units="KiB" cpus="2"/>
      <bank type="l1d" size="256" units="KiB" cpus="3"/>
      <bank type="l1d" size="256" units="KiB" cpus="4"/>
      <bank type="l1d" size="256" units="KiB" cpus="5"/>
      <bank type="l1d" size="256" units="KiB" cpus="6"/>
      <bank type="l1d" size="256" units="KiB" cpus="7"/>
      <bank type="l1d" size="256" units="KiB" cpus="8"/>
      <bank type="l1d" size="256" units="KiB" cpus="9"/>
      <bank type="l1d" size="256" units="KiB" cpus="10"/>
      <bank type="l1d" size="256" units="KiB" cpus="11"/>
    </cache>

which shows each socket has its own dedicated L3 cache, and each
core has its own L2 & L1 cache.

> 2. Extend capabilities outputs.
> 
> virsh capabilities | grep resctrl
>     <cpu>
>     ...
>       <resctrl name='L3' unit='KiB' cache_size='56320' cache_unit='2816'/>
>     </cpu>
> 
>     This will tell that the host have enabled resctrl(which you can find it in /sys/fs/resctrl),
> And it supports to allocate 'L3' type cache, total 'L3' cache size is 56320 KiB, and the minimum unit size of 'L3' cache is 2816 KiB.
>   P.S. L3 cache size unit is the minum l3 cache unit can be allocated. It's hardware related and can not be changed.

If we're already reported cache in the capabilities from step
one, then it ought to be extendable to cover this reporting.

    <cache>
      <bank type="l3" size="56320" units="KiB" cpus="0,2,3,6,7,8">
          <control unit="KiB" min="2816"/>
      </bank>
      <bank type="l3" size="56320" units="KiB" cpus="3,4,5,9,10,11">
          <control unit="KiB" min="2816"/>
      </bank>
    </cache>

note how we report the control info for both l3 caches, since they
come from separate sockets and thus could conceivably report different
info if different CPUs were in each socket.

> 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
> 
> for example I have a 2 socket cpus host, and I'v enabled cat_l3 feature only
> 
> root at s2600wt:~/linux# virsh nodecachestats
> L3.0 : 56320 KiB
> L3.1 : 56320 KiB
> 
>   P.S. resource_type can be L3, L3DATA, L3CODE, L2 for now.

This feels like something we should have in the capabilities XML too
rather than a new command

    <cache>
      <bank type="l3" size="56320" units="KiB" cpus="0,2,3,6,7,8">
          <control unit="KiB" min="2816" avail="56320/>
      </bank>
      <bank type="l3" size="56320" units="KiB" cpus="3,4,5,9,10,11">
          <control unit="KiB" min="2816" avail="56320"/>
      </bank>
    </cache>

> 4. Add new interface to manage how many cache can be allociated for a domain
> 
> root at s2600wt:~/linux# virsh cachetune kvm02 --l3.count 2
> 
> root at s2600wt:~/linux# virsh cachetune kvm02
> l3.count       : 2
> 
> This will allocate 2 units(2816 * 2) l3 cache for domain kvm02
> 
> ## Domain XML changes
> 
> Cache Tuneing
> 
> <domain>
>   ...
>   <cachetune>
>     <l3_cache_count>2</l3_cache_count>
>   </cachetune>
>   ...
> </domain>

IIUC, the kernel lets us associate individual PIDs
with each cache. Since each vCPU is a PID, this means
we are able to allocate different cache size to
different CPUs. So we need to be able to represent
that in the XML. I think we should also represent
the allocation in a normal size (ie KiB), not in
count of min unit.

So eg this shows allocating two cache banks and giving
one to the first 4 cpus, and one to the second 4 cpus

   <cachetune>
      <bank type="l3" size="5632" unit="KiB" cpus="0,1,2,3"/>
      <bank type="l3" size="5632" unit="KiB" cpus="4,5,6,7"/>
   </cachetune>


Regards,
Daniel
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