Re: [libvirt] [PATCH 7/9] perf: add page_faults_maj software perf event support

On Mon, Feb 13, 2017 at 8:14 PM, John Ferlan <jferlan(a)redhat.com <mailto:jferlan@redhat.com>> wrote: On 02/13/2017 01:49 AM, Nitesh Konkar wrote: > > > > On Fri, Feb 10, 2017 at 3:22 AM, John Ferlan <jferlan(a)redhat.com <mailto:jferlan@redhat.com> > <mailto:jferlan@redhat.com <mailto:jferlan@redhat.com>>> wrote: > > > > On 01/27/2017 06:01 AM, Nitesh Konkar wrote: > > This patch adds support and documentation > > for the page_faults_maj perf event. > > > > Signed-off-by: Nitesh Konkar <nitkon12(a)linux.vnet.ibm.com <mailto:nitkon12@linux.vnet.ibm.com> <mailto:nitkon12@linux.vnet.ibm.com <mailto:nitkon12@linux.vnet.ibm.com>>> > > --- > > docs/formatdomain.html.in <http://formatdomain.html.in> <http://formatdomain.html.in> > | 7 +++++++ > > docs/news.xml | 4 ++-- > > docs/schemas/domaincommon.rng | 1 + > > include/libvirt/libvirt-domain.h | 10 ++++++++++ > > src/libvirt-domain.c | 3 +++ > > src/qemu/qemu_driver.c | 1 + > > src/util/virperf.c | 5 ++++- > > src/util/virperf.h | 1 + > > tests/genericxml2xmlindata/generic-perf.xml | 1 + > > tools/virsh.pod | 5 +++++ > > 10 files changed, 35 insertions(+), 3 deletions(-) > > > > NB: Similar comments from the page_faults_min... > > > diff --git a/docs/formatdomain.html.in <http://formatdomain.html.in> <http://formatdomain.html.in> > b/docs/formatdomain.html.in <http://formatdomain.html.in> <http://formatdomain.html.in> > > index 1857168..50a6bdb 100644 > > --- a/docs/formatdomain.html.in <http://formatdomain.html.in> <http://formatdomain.html.in> > > +++ b/docs/formatdomain.html.in <http://formatdomain.html.in> <http://formatdomain.html.in> > > @@ -1943,6 +1943,7 @@ > > &lt;event name='context_switches' enabled='no'/&gt; > > &lt;event name='cpu_migrations' enabled='no'/&gt; > > &lt;event name='page_faults_min' enabled='no'/&gt; > > + &lt;event name='page_faults_maj' enabled='no'/&gt; > > &lt;/perf&gt; > > ... > > </pre> > > @@ -2052,6 +2053,12 @@ > > platform</td> > > <td><code>perf.page_faults_min</code></td> > > </tr> > > + <tr> > > + <td><code>page_faults_maj</code></td> > > + <td>the count of major page faults by applications running on the > > + platform</td> > > + <td><code>perf.page_faults_maj</code></td> > > + </tr> > > As already noted in patch 3... is maj+min the same as what patch 3 > provides? > > > maj+min is not always exactly the same as page faults. Sometimes there > is a small offset > value. > > Eg: perf record -a --event={page-faults,major-faults,minor-faults} > 47 > page-faults > > 0 > major-faults > > 46 minor-faults > Offset by 1 > > Eg: virsh domstats --perf > Domain: 'Fedora' > perf.page_faults=890 > perf.page_faults_min=890 > perf.page_faults_maj=0 > Here maj+min=page_faults > > Thus are all necessary? > > I am not sure on this part. Probably yes as we dont want > the user to add min+maj to get (approx)total page faults. > > I don't mind all 3 being present... still if I'm going to ask the question, then someone getting the stats will ask the question... they may also wonder why maj+min != total. Perhaps something you could dig deeper on with the kernel code descriptions that are setting the value. My assumption is it's the "time" of the sample. That is a total page fault could have been counted even though it hadn't been counted as a maj or min page fault. I looked into the kernel code in /arch/x86/mm/fault.c and also confirmed from the #perf IRC that maj+min != total is valid. This is because not all page faults fall in maj/min category. Some maybe invalid page faults(invalid address generated) whereas some pagefaults after occuring are not serviced due to lock contention so as to avoid a deadlock at that instance, thus being counted in total but not in min/maj faults.

Also, shd i follow the comment pattern as shown in ur patch under review, in /virsh.pod ?