Re: [PATCH v3 3/3] Add support for RAPL MSRs in KVM/Qemu

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Mar 13, 2024 at 11:48:19AM +0100, Anthony Harivel wrote:
> Hi Daniel,
> 
> Daniel P. Berrangé, Mar 12, 2024 at 16:49:
> 
> > The point still stands though. NUMA node ID numbers are not
> > guaranteed to be the same as socket ID numbers. Very often
> > then will be the same (which makes it annoying to test as it
> > is easy to not realize the difference), but we can't rely on
> > that.
> >
> > > I'm using functions of libnuma to populate the maxpkgs of the host. 
> > > I tested this on different Intel CPU with multiple packages and this 
> > > has always returned the good number of packages. A false positive ?
> >
> > maxpkgs comes from vmsr_get_max_physical_package() which you're
> > reading from sysfs, rather than libnuma.
> >
> > > So here I'm checking if the thread has run on the package number 'i'. 
> > > I populate 'numa_node_id' with numa_node_of_cpu().
> > > 
> > > I did not wanted to reinvent the wheel and the only lib that was talking 
> > > about "node" was libnuma.
> >
> > I'm not actually convinced we need to use libnuma at all. IIUC, you're
> > just trying to track all CPUs within the same physical socket (package).
> > I don't think we need to care about NUMA nodes to do that tracking.
> >
> 
> Alright, having a deeper look I'm actually using NUMA for 2 info:
> 
> - How many cpu per Package: this helps me calculate the ratio.
> 
> - To whom package the cpu belongs: to calculate the ratio with the right 
>   package energy counter.
> 
> Without libnuma, I'm bit confused on how to handle this. 
> 
> Should I parse /sys/bus/node/devices/node* to know how many packages ?
> Should I parse /sys/bus/node/devices/node0/cpu0/topology/core_cpus_list 
> to handle which cpu belongs to which package ?

You don't need to access it via the /node/ hierarchy

The canonical path for CPUs would be

  /sys/devices/system/cpu/cpuNNN/topology

The core_cpus_list file is giving you hyper-thread siblings within
a core, which I don't think is what you want.

If you're after discrete physical packages, then 'package_cpus_list'
gives you all CPUs within a physical socket (package) I believe.

> Would that be too cumbusome for the user to enter the detail about how
> many packages and how many cpu per pakages ? 
> 
> i.e: 
> -kvm,rapl=true,maxpkgs=2,cpupkgs=8,rapl-helper-socket=/path/sock.sock

That won't cope with asymmetrical CPU configurations, so I think it
is preferrable to read the info from sysfs.

With regards,
Daniel
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