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

> -----Original Message----- > From: Daniel P. Berrange [mailto:berrange@redhat.com] > Sent: Monday, July 20, 2015 5:32 PM > To: Ren, Qiaowei > Cc: libvir-list(a)redhat.com > Subject: Re: [libvirt] [PATCH 2/3] Qemu: add CMT support > > On Sun, Jul 05, 2015 at 07:43:43PM +0800, Qiaowei Ren 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. > > > > Signed-off-by: Qiaowei Ren <qiaowei.ren(a)intel.com&gt; > > > diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index > > 4cfae03..8c678c9 100644 > > --- a/src/qemu/qemu_driver.c > > +++ b/src/qemu/qemu_driver.c > > @@ -19320,6 +19320,53 @@ qemuDomainGetStatsBlock(virQEMUDriverPtr > > driver, > > > > #undef QEMU_ADD_COUNT_PARAM > > > > +static int > > +qemuDomainGetStatsCache(virQEMUDriverPtr driver ATTRIBUTE_UNUSED, > > + virDomainObjPtr dom, > > + virDomainStatsRecordPtr record, > > + int *maxparams, > > + unsigned int privflags ATTRIBUTE_UNUSED) > > So this is a method that is used to collect per-domain information > > > +{ > > + qemuDomainObjPrivatePtr priv = dom->privateData; > > + FILE *fd; > > + unsigned long long cache = 0; > > + int scaling_factor = 0; > > + > > + if (priv->cmt_fd <= 0) > > + return -1; > > + > > + if (read(priv->cmt_fd, &cache, sizeof(uint64_t)) < 0) { > > + virReportSystemError(errno, "%s", > > + _("Unable to read cache data")); > > + return -1; > > + } > > + > > + fd = fopen("/sys/devices/intel_cqm/events/llc_occupancy.scale", "r"); > > + if (!fd) { > > + virReportSystemError(errno, "%s", > > + _("Unable to open CMT scale file")); > > + return -1; > > + } > > + if (fscanf(fd, "%d", &scaling_factor) != 1) { > > + virReportSystemError(errno, "%s", > > + _("Unable to read CMT scale file")); > > + VIR_FORCE_FCLOSE(fd); > > + return -1; > > + } > > + VIR_FORCE_FCLOSE(fd); > > But this data you are reading is global to the entire host. > In fact this data is only for per-domain. When the perf syscall is called to enable perf event for domain, the pid of that domain is passed.

> > -5122,6 +5199,15 @@ void qemuProcessStop(virQEMUDriverPtr driver, > > virPortAllocatorRelease(driver->migrationPorts, priv->nbdPort); > > priv->nbdPort = 0; > > > > + /* Disable CMT */ > > + if (priv->cmt_fd > 0) { > > You can't rely on keeping an open file descriptor for the guest because libvirtd > may be restarted. > Sorry, I don't really get the meaning of this. You mean that when libvirtd is restarted, those resource which the domain opened should be closed, right?

> > + if (ioctl(priv->cmt_fd, PERF_EVENT_IOC_DISABLE) < 0) { > > + virReportSystemError(errno, "%s", > > + _("Unable to disable perf event for CMT")); > > + } > > + VIR_FORCE_CLOSE(priv->cmt_fd); > > + } > > + > > if (priv->agent) { > > qemuAgentClose(priv->agent); > > priv->agent = NULL; > > > Conceptually I think this approach to implementation is flawed. While you are > turning on/off the perf events for each QEMU process, the data collection does > not distinguish data from each QEMU process - the data reported is host wide. > So this really doesn't make much sense IMHO. > As mentioned above, the data reported is only for domain. > I'm also wondering whether this is really going to be sufficiently useful on its > own. CPUs have countless other performance counters that I would imagine > apps/admins will want to read in order to analyse QEMU performance, beyond > this new CMT feature. The domain stats API won't really scale up to dealing with > arbitrary perf event counter reporting so I'm not much of a fan of just special > casing CMT in this way. > > IOW, if we want to support host performance analysis in libvirt, then we > probably want to design an set of APIs specifically for this purpose, but I could > well see us saying that this is out of scope for libvirt and apps shoud just use the > linux perf interfaces directly. > Yes. I can get what you mean. Maybe libvirt doesn't have to be responsible for supporting host performance. But I guess cache usage should be important for each domain, if those apps based on libvirt can achieve this information they will be able to better check and confirm the domain works normally, like the stats of cpu/memory/block/... which have be supported in libvirt now. Do you think so?