Messages in this thread | | | From | Namhyung Kim <> | Date | Tue, 2 Nov 2021 16:21:21 -0700 | Subject | Re: [PATCH v3] perf evsel: Fix missing exclude_{host,guest} setting |
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Hi Jiri,
On Tue, Nov 2, 2021 at 7:10 AM Jiri Olsa <jolsa@redhat.com> wrote: > > On Fri, Oct 29, 2021 at 03:49:29PM -0700, Namhyung Kim wrote: > > The current logic for the perf missing feature has a bug that it can > > wrongly clear some modifiers like G or H. Actually some PMUs don't > > support any filtering or exclusion while others do. But we check it > > as a global feature. > > > > For example, the cycles event can have 'G' modifier to enable it only > > in the guest mode on x86. When you don't run any VMs it'll return 0. > > > > # perf stat -a -e cycles:G sleep 1 > > > > Performance counter stats for 'system wide': > > > > 0 cycles:G > > > > 1.000721670 seconds time elapsed > > > > But when it's used with other pmu events that don't support G modifier, > > it'll be reset and return non-zero values. > > > > # perf stat -a -e cycles:G,msr/tsc/ sleep 1 > > > > Performance counter stats for 'system wide': > > > > 538,029,960 cycles:G > > 16,924,010,738 msr/tsc/ > > > > 1.001815327 seconds time elapsed > > > > This is because of the missing feature detection logic being global. > > Add a hashmap to set pmu-specific exclude_host/guest features. > > > > Reported-by: Stephane Eranian <eranian@google.com> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org> > > --- > > v3 changes) > > * check memory allocation failure > > * add more NULL check > > we were discussing this with Arnaldo yesterday and he had an idea to use > evsel->pmu link to store this info instead of hash.. I first thought we > needed 'evsel' related data, but after I gave it some thought I think that > might actually work
I don't get it.. do we have evsel->pmu already? Or do you want to add it? Yeah, the filtering facility (attr.exclude_*) should be kept in a PMU data not in the evsel. So I added a hashmap to find the pmu data from attr.type. How do I use evsel->pmu to store the info then?
> > my argument was following usecase: > > cycles:G,instructions:G,pmu/bla1/:G,pmu/bla2/ > > that we would falsely clear pmu/bla1/:G if we used the 'evsel->pmu' data.. > but then I realized it's detection if pmu support :G and so if the :G is > not there, none of the events should have it > > thoughts?
I don't think I'm following well... ;-p
If the pmu doesn't support host/guest filtering, pmu/bla1/G may count something. Not sure if it's better to error out. But the cycles:G and instructions:G should result in 0 in case there's no VM running.
Thanks, Namhyung
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