Re: [PATCH v3 16/35] perf pmu: Remove perf_pmu__hybrid_mounted

[Ian Rogers]

On Fri, May 26, 2023 at 12:02 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2023-05-26 2:33 p.m., Ian Rogers wrote:
> > On Fri, May 26, 2023 at 11:14 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2023-05-24 6:18 p.m., Ian Rogers wrote:
> >>> perf_pmu__hybrid_mounted is used to detect whether cpu_core or
> >>> cpu_atom is mounted with a non-empty cpus file by
> >>> pmu_lookup. pmu_lookup will attempt to read the cpus file too and so
> >>> the check can be folded into this.
> >>>
> >>> Checking hybrid_mounted in pmu_is_uncore is redundant as the next
> >>> cpumask read will fail returning false.
> >>>
> >>> Reduce the scope of perf_pmu__find_hybrid_pmu by making it static.
> >>>
> >>> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> >>> ---
> >>>  tools/perf/util/pmu-hybrid.c | 15 +--------------
> >>>  tools/perf/util/pmu-hybrid.h |  3 ---
> >>>  tools/perf/util/pmu.c        | 26 ++++++++++++++------------
> >>>  3 files changed, 15 insertions(+), 29 deletions(-)
> >>>
> >>> diff --git a/tools/perf/util/pmu-hybrid.c b/tools/perf/util/pmu-hybrid.c
> >>> index bc4cb0738c35..7fe943dd3217 100644
> >>> --- a/tools/perf/util/pmu-hybrid.c
> >>> +++ b/tools/perf/util/pmu-hybrid.c
> >>> @@ -18,20 +18,7 @@
> >>>
> >>>  LIST_HEAD(perf_pmu__hybrid_pmus);
> >>>
> >>> -bool perf_pmu__hybrid_mounted(const char *name)
> >>> -{
> >>> -     int cpu;
> >>> -     char pmu_name[PATH_MAX];
> >>> -     struct perf_pmu pmu = {.name = pmu_name};
> >>> -
> >>> -     if (strncmp(name, "cpu_", 4))
> >>> -             return false;
> >>> -
> >>> -     strlcpy(pmu_name, name, sizeof(pmu_name));
> >>> -     return perf_pmu__scan_file(&pmu, "cpus", "%u", &cpu) > 0;
> >>> -}
> >>> -
> >>> -struct perf_pmu *perf_pmu__find_hybrid_pmu(const char *name)
> >>> +static struct perf_pmu *perf_pmu__find_hybrid_pmu(const char *name)
> >>>  {
> >>>       struct perf_pmu *pmu;
> >>>
> >>> diff --git a/tools/perf/util/pmu-hybrid.h b/tools/perf/util/pmu-hybrid.h
> >>> index 206b94931531..8dbcae935020 100644
> >>> --- a/tools/perf/util/pmu-hybrid.h
> >>> +++ b/tools/perf/util/pmu-hybrid.h
> >>> @@ -13,9 +13,6 @@ extern struct list_head perf_pmu__hybrid_pmus;
> >>>  #define perf_pmu__for_each_hybrid_pmu(pmu)   \
> >>>       list_for_each_entry(pmu, &perf_pmu__hybrid_pmus, hybrid_list)
> >>>
> >>> -bool perf_pmu__hybrid_mounted(const char *name);
> >>> -
> >>> -struct perf_pmu *perf_pmu__find_hybrid_pmu(const char *name);
> >>>  bool perf_pmu__is_hybrid(const char *name);
> >>>
> >>>  static inline int perf_pmu__hybrid_pmu_num(void)
> >>> diff --git a/tools/perf/util/pmu.c b/tools/perf/util/pmu.c
> >>> index cd94abe7a87a..e9f3e6a777c0 100644
> >>> --- a/tools/perf/util/pmu.c
> >>> +++ b/tools/perf/util/pmu.c
> >>> @@ -617,9 +617,6 @@ static bool pmu_is_uncore(int dirfd, const char *name)
> >>>  {
> >>>       int fd;
> >>>
> >>> -     if (perf_pmu__hybrid_mounted(name))
> >>> -             return false;
> >>> -
> >>>       fd = perf_pmu__pathname_fd(dirfd, name, "cpumask", O_PATH);
> >>>       if (fd < 0)
> >>>               return false;
> >>> @@ -900,6 +897,16 @@ static int pmu_max_precise(int dirfd, struct perf_pmu *pmu)
> >>>       return max_precise;
> >>>  }
> >>>
> >>> +/**
> >>> + * perf_pmu__skip_empty_cpus() - should pmu_lookup skip the named PMU if the
> >>> + *      cpus or cpumask file isn't present?
> >>> + * @name: Name of PMU.
> >>> + */
> >>> +static bool perf_pmu__skip_empty_cpus(const char *name)
> >>> +{
> >>> +     return !strcmp(name, "cpu_core") || !strcmp(name, "cpu_atom");
> >>
> >> Can we use the below to replace?
> >> return !strncmp(name, "cpu_", 4);
> >>
> >> Otherwise, anytime a new core PMU name is introduced, I have to patch
> >> the function.
> >
> > I dislike this function but was carrying it forward, I think we can
> > get rid of it. The point of erroring is to not have core PMUs when
> > there are no online CPUs associated with it. For existing core PMUs
> > this just isn't something that can happen as otherwise what CPU are
> > you running on. For hybrid it can happen and we know we care because
> > the PMU's type is core.
>
> For hybrid, I think it can only happen when there is a kernel bug, e.g.,
> a new core PMU is added but forgets to set the CPU mask.
>
> > So why not change the error to be when the cpu
> > map is empty and the CPU is core?
>
> I don't think PT has cpu map. Other PMUs, e.g., msr, don't have cpu map
> either. They are not core PMU.
>
> Actually, I'm OK with just removing the function. Maybe we can add a
> test to check the CPU mask on hybrid. If it doesn't exist, it should be
> a bug of perf.
>
> Thanks,
> Kan

So having no CPUs but a hybrid PMU does strike me as strange. The
original commit messages describe things like unmounting PMUs as being
a motivation. I'll go ahead and remove the code entirely for now. When
we find the bug it was supposed to be addressing we can add something
like a core check back in and add a test :-)

Thanks,
Ian

> > I'm going to assume that my logic is
> > sound and change the code in v4, but please complain.
> >
> > Thanks,
> > Ian
> >
> >> Thanks,
> >> Kan
> >>
> >>> +}
> >>> +
> >>>  static struct perf_pmu *pmu_lookup(int dirfd, const char *lookup_name)
> >>>  {
> >>>       struct perf_pmu *pmu;
> >>> @@ -907,15 +914,8 @@ static struct perf_pmu *pmu_lookup(int dirfd, const char *lookup_name)
> >>>       LIST_HEAD(aliases);
> >>>       __u32 type;
> >>>       char *name = pmu_find_real_name(lookup_name);
> >>> -     bool is_hybrid = perf_pmu__hybrid_mounted(name);
> >>>       char *alias_name;
> >>>
> >>> -     /*
> >>> -      * Check pmu name for hybrid and the pmu may be invalid in sysfs
> >>> -      */
> >>> -     if (!strncmp(name, "cpu_", 4) && !is_hybrid)
> >>> -             return NULL;
> >>> -
> >>>       /*
> >>>        * The pmu data we store & need consists of the pmu
> >>>        * type value and format definitions. Load both right
> >>> @@ -935,8 +935,10 @@ static struct perf_pmu *pmu_lookup(int dirfd, const char *lookup_name)
> >>>               return NULL;
> >>>
> >>>       pmu->cpus = pmu_cpumask(dirfd, name);
> >>> -     pmu->name = strdup(name);
> >>> +     if (!pmu->cpus && perf_pmu__skip_empty_cpus(name))
> >>> +             goto err;
> >>>
> >>> +     pmu->name = strdup(name);
> >>>       if (!pmu->name)
> >>>               goto err;
> >>>
> >>> @@ -967,7 +969,7 @@ static struct perf_pmu *pmu_lookup(int dirfd, const char *lookup_name)
> >>>       list_splice(&aliases, &pmu->aliases);
> >>>       list_add_tail(&pmu->list, &pmus);
> >>>
> >>> -     if (is_hybrid)
> >>> +     if (!strcmp(name, "cpu_core") || !strcmp(name, "cpu_atom"))
> >>>               list_add_tail(&pmu->hybrid_list, &perf_pmu__hybrid_pmus);
> >>>       else
> >>>               INIT_LIST_HEAD(&pmu->hybrid_list);