Re: [PATCH 8/8] perf tools: Check fallback error and order

[Namhyung Kim]

On Wed, Sep 04, 2024 at 09:19:25AM -0700, Ian Rogers wrote:
> On Tue, Sep 3, 2024 at 11:41 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > The perf_event_open might fail due to various reasons, so blindly
> > reducing precise_ip level might not be the best way to deal with it.
> >
> > It seems the kernel return -EOPNOTSUPP when PMU doesn't support the
> > given precise level.  Let's try again with the correct error code.
> 
> We also have pmu's max_precise:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.h?h=perf-tools-next#n91
> The reducing the precision approach was iirc taken for AMD who will
> forward some precise events to IBS, but the max_precise on the cpu PMU
> is 0. I think because of this, reducing the precision below
> evsel->pmu->max_precise shouldn't be necessary and another fallback
> may help better.

Internally IBS has max_precise of 2 and I think it should have that in
the sysfs.

But I found a problem with this code.  Now cycles:P would stop at 2
because after that it won't return EOPNOTSUPP.  Instead, it returns
EINVAL because of exclude_kernel and PERF_PMU_CAP_NO_EXCLUDE.

Maybe we need something like this.. :(

Thanks,
Namhyung


diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 0133c9ad3ce07a24..6157dc68044eb389 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2587,6 +2587,13 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
        if (err == -EINVAL && evsel__detect_missing_features(evsel))
                goto fallback_missing_features;
 
+       /* HACK: AMD IBS doesn't accept exclude_*, forwarding it back to core PMU */
+       if (err == -EINVAL && evsel->precise_max && evsel->core.attr.precise_ip &&
+                       evsel->core.attr.exclude_kernel) {
+               evsel->core.attr.precise_ip = 0;
+               goto fallback_missing_features;
+       }
+
 out_close:
        if (err)
                threads->err_thread = thread;