Re: [PATCH 1/2] perf tool AMD: Use non-precise cycles as default event on certain Zen2 processors

[Ian Rogers]

On Tue, Nov 7, 2023 at 12:34 AM Ravi Bangoria <ravi.bangoria@xxxxxxx> wrote:
>
> By default, Perf uses precise cycles event when no explicit event is
> specified by user. Precise cycles event is forwarded to ibs_op// pmu
> on AMD. However, IBS has hw issue on certain Zen2 processors where
> it might raise NMI without sample_valid bit set, which causes Unknown
> NMI warnings. (Erratum #1215: IBS (Instruction Based Sampling) Counter
> Valid Value May be Incorrect After Exit From Core C6 (CC6) State.) So,
> use non-precise cycles as default event on affected processors.
>
> This does not prevent user to use explicit precise cycles event or
> ibs_op// pmu directly.
>
> Suggested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>

Thanks Ravi,

We read max_precise from sysfs:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.c?h=perf-tools-next#n984

But it appears not to be used:
https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/evsel.c?h=perf-tools-next#n1323

I think:
```
if (evsel->precise_max)
    attr->precise_ip = 3;
```
should be:
```
if (evsel->precise_max)
    attr->precise_ip = evsel->pmu->max_precise;
```

Presumably this is misreporting as some non-zero value on the buggy
systems - if it doesn't we can declare victory here. If not we can
modify max_precise's calculation on perf's struct pmu for cpu for AMD
so that we just hardwire it to zero in arch code.

I think I prefer this approach as it is fixing max_precise in a more
generic way. If someone were to specify "cycles:P" on the command line
then it wouldn't trigger the bug, similarly for perf script and other
places. Wdyt?

Thanks,
Ian

> ---
>  tools/perf/arch/x86/util/evlist.c | 34 +++++++++++++++++++++++++++++++
>  tools/perf/builtin-record.c       |  2 +-
>  tools/perf/builtin-top.c          |  2 +-
>  tools/perf/util/evlist.c          | 12 ++++++++++-
>  tools/perf/util/evlist.h          |  2 ++
>  5 files changed, 49 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/arch/x86/util/evlist.c b/tools/perf/arch/x86/util/evlist.c
> index b1ce0c52d88d..f4478179c91b 100644
> --- a/tools/perf/arch/x86/util/evlist.c
> +++ b/tools/perf/arch/x86/util/evlist.c
> @@ -5,6 +5,8 @@
>  #include "util/evlist.h"
>  #include "util/parse-events.h"
>  #include "util/event.h"
> +#include "util/env.h"
> +#include "linux/string.h"
>  #include "topdown.h"
>  #include "evsel.h"
>
> @@ -92,3 +94,35 @@ int arch_evlist__cmp(const struct evsel *lhs, const struct evsel *rhs)
>         /* Default ordering by insertion index. */
>         return lhs->core.idx - rhs->core.idx;
>  }
> +
> +/*
> + * Precise cycles event is forwarded to ibs_op// pmu on AMD. However, IBS
> + * has hw issue on certain Zen2 processors where it might raise NMI without
> + * sample_valid bit set, which causes Unknown NMI warnings. So default to
> + * non-precise cycles event on affected processors.
> + */
> +const char *arch_evlist__default_cycles_event(bool can_profile_kernel)
> +{
> +       struct perf_env env = { .total_mem = 0, };
> +       unsigned int family, model, stepping;
> +       bool is_amd;
> +       int ret;
> +
> +       perf_env__cpuid(&env);
> +       is_amd = env.cpuid && strstarts(env.cpuid, "AuthenticAMD");
> +       if (!is_amd)
> +               goto out;
> +
> +       ret = sscanf(env.cpuid, "%*[^,],%u,%u,%u", &family, &model, &stepping);
> +       if (ret == 3 && family == 0x17 && (
> +           (model >= 0x30 && model <= 0x3f) ||
> +           (model >= 0x60 && model <= 0x7f) ||
> +           (model >= 0x90 && model <= 0x9f))) {
> +               perf_env__exit(&env);
> +               return can_profile_kernel ? "cycles" : "cycles:u";
> +       }
> +
> +out:
> +       perf_env__exit(&env);
> +       return can_profile_kernel ? "cycles:P" : "cycles:Pu";
> +}
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index dcf288a4fb9a..e58d8ac8a77b 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -4150,7 +4150,7 @@ int cmd_record(int argc, const char **argv)
>         if (rec->evlist->core.nr_entries == 0) {
>                 bool can_profile_kernel = perf_event_paranoid_check(1);
>
> -               err = parse_event(rec->evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +               err = parse_event(rec->evlist, evlist__default_cycles_event(can_profile_kernel));
>                 if (err)
>                         goto out;
>         }
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index ea8c7eca5eee..21368421eddd 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -1666,7 +1666,7 @@ int cmd_top(int argc, const char **argv)
>
>         if (!top.evlist->core.nr_entries) {
>                 bool can_profile_kernel = perf_event_paranoid_check(1);
> -               int err = parse_event(top.evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +               int err = parse_event(top.evlist, evlist__default_cycles_event(can_profile_kernel));
>
>                 if (err)
>                         goto out_delete_evlist;
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index e36da58522ef..406ed851cafc 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -90,6 +90,16 @@ struct evlist *evlist__new(void)
>         return evlist;
>  }
>
> +const char * __weak arch_evlist__default_cycles_event(bool can_profile_kernel)
> +{
> +       return can_profile_kernel ? "cycles:P" : "cycles:Pu";
> +}
> +
> +const char *evlist__default_cycles_event(bool can_profile_kernel)
> +{
> +       return arch_evlist__default_cycles_event(can_profile_kernel);
> +}
> +
>  struct evlist *evlist__new_default(void)
>  {
>         struct evlist *evlist = evlist__new();
> @@ -100,7 +110,7 @@ struct evlist *evlist__new_default(void)
>                 return NULL;
>
>         can_profile_kernel = perf_event_paranoid_check(1);
> -       err = parse_event(evlist, can_profile_kernel ? "cycles:P" : "cycles:Pu");
> +       err = parse_event(evlist, evlist__default_cycles_event(can_profile_kernel));
>         if (err) {
>                 evlist__delete(evlist);
>                 evlist = NULL;
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 98e7ddb2bd30..7267b4fb1981 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -91,6 +91,8 @@ struct evsel_str_handler {
>
>  struct evlist *evlist__new(void);
>  struct evlist *evlist__new_default(void);
> +const char *arch_evlist__default_cycles_event(bool can_profile_kernel);
> +const char *evlist__default_cycles_event(bool can_profile_kernel);
>  struct evlist *evlist__new_dummy(void);
>  void evlist__init(struct evlist *evlist, struct perf_cpu_map *cpus,
>                   struct perf_thread_map *threads);
> --
> 2.41.0
>