Re: [libvirt] [PATCH 2/3] Qemu: add CMT support

On Jul 6, 2015 14:49, Prerna wrote: > On Sun, Jul 5, 2015 at 5:13 PM, Qiaowei Ren <qiaowei.ren(a)intel.com > <mailto:qiaowei.ren@intel.com> > wrote: > > > One RFC in > https://www.redhat.com/archives/libvir-list/2015-June/msg01509.html > > CMT (Cache Monitoring Technology) can be used to measure the > usage of cache by VM running on the host. This patch will > extend the bulk stats API (virDomainListGetStats) to add this > field. Applications based on libvirt can use this API to achieve > cache usage of VM. Because CMT implementation in Linux kernel > is based on perf mechanism, this patch will enable perf event > for CMT when VM is created and disable it when VM is destroyed. > > > > > Hi Ren, > > One query wrt this implementation. I see you make a perf ioctl to > gather CMT stats each time the stats API is invoked. > > If the CMT stats are exposed by a hardware counter, then this implies > logging on a per-cpu (or per-socket ???) basis. > > This also implies that the value read will vary as the CPU (or socket) > on which it is being called changes. > > > Now, with this background, if we need real-world stats on a VM, we need > this perf ioctl executed on all CPUs/ sockets on which the VM ran. > Also, once done, we will need to aggregate results from each of these > sources. > > > In this implementation, I am missing this -- there seems no control > over which physical CPU the libvirt worker thread will run and collect > the perf data from. Data collected from this implementation might not > accurately model the system state. > > I _think_ libvirt currently has no way of directing a worker thread to > collect stats from a given CPU -- if we do, I would be happy to learn > about it :) > Prerna, thanks for your reply. I checked the CMT implementation in kernel, and noticed that the series implement new ->count() of pmu driver which can aggregate the results from each cpu if perf type is PERF_TYPE_INTEL_CQM . The following is the link for the patch: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?i d=bfe1fc d2688f557a6b6a88f59ea7619228728bd7 So I guess that this patch just need to set right perf type and "cpu=-1". Do you think this is ok? Thanks, Qiaowei