Re: [PATCH 2/3] perf tools: Fix SMT not detected with large core count

[Ian Rogers]

On Tue, Nov 23, 2021 at 3:34 PM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Tue, Nov 23, 2021 at 02:48:20PM -0800, Ian Rogers escreveu:
> > sysfs__read_int returns 0 on success, and so the fast read path was
> > always failing.
>
> Please split this into two patches, the above part should be in one, and
> the strtoull in another.

Will do.

> Also can't we just do as ./tools/perf/util/cputopo.c and use instead
> core_cpus_list?

It is more complex for the list case as the list may have a range. It
shouldn't really matter with the active fix in any case. I think
ideally we'd have an abstraction like cpuset in util-linux:
https://github.com/util-linux/util-linux/blob/master/lib/cpuset.c
This is a bit beyond what I'm trying to fix here.

Thanks,
Ian

> On a 5950x:
>
> ⬢[acme@toolbox perf]$ cat /sys/devices/system/cpu/cpu31/topology/core_cpus
> 80008000
> ⬢[acme@toolbox perf]$ cat /sys/devices/system/cpu/cpu31/topology/core_cpus_list
> 15,31
> ⬢[acme@toolbox perf]$ cat /sys/devices/system/cpu/cpu0/topology/core_cpus
> 00010001
> ⬢[acme@toolbox perf]$ cat /sys/devices/system/cpu/cpu0/topology/core_cpus_list
> 0,16
> ⬢[acme@toolbox perf]$
>
> - Arnaldo
>
> > strtoull can only read a 64-bit bitmap. On an AMD EPYC core_cpus may look
> > like:
> > 00000000,00000000,00000000,00000001,00000000,00000000,00000000,00000001
> > and so the sibling wasn't spotted. Fix by writing a simple hweight string
> > parser.
> >
> > Fixes: bb629484d924 (perf tools: Simplify checking if SMT is active.)
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/util/smt.c | 68 ++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 58 insertions(+), 10 deletions(-)
> >
> > diff --git a/tools/perf/util/smt.c b/tools/perf/util/smt.c
> > index 20bacd5972ad..2636be65305a 100644
> > --- a/tools/perf/util/smt.c
> > +++ b/tools/perf/util/smt.c
> > @@ -5,6 +5,56 @@
> >  #include "api/fs/fs.h"
> >  #include "smt.h"
> >
> > +/**
> > + * hweight_str - Returns the number of bits set in str. Stops at first non-hex
> > + *          or ',' character.
> > + */
> > +static int hweight_str(char *str)
> > +{
> > +     int result = 0;
> > +
> > +     while (*str) {
> > +             switch (*str++) {
> > +             case '0':
> > +             case ',':
> > +                     break;
> > +             case '1':
> > +             case '2':
> > +             case '4':
> > +             case '8':
> > +                     result++;
> > +                     break;
> > +             case '3':
> > +             case '5':
> > +             case '6':
> > +             case '9':
> > +             case 'a':
> > +             case 'A':
> > +             case 'c':
> > +             case 'C':
> > +                     result += 2;
> > +                     break;
> > +             case '7':
> > +             case 'b':
> > +             case 'B':
> > +             case 'd':
> > +             case 'D':
> > +             case 'e':
> > +             case 'E':
> > +                     result += 3;
> > +                     break;
> > +             case 'f':
> > +             case 'F':
> > +                     result += 4;
> > +                     break;
> > +             default:
> > +                     goto done;
> > +             }
> > +     }
> > +done:
> > +     return result;
> > +}
> > +
> >  int smt_on(void)
> >  {
> >       static bool cached;
> > @@ -15,9 +65,12 @@ int smt_on(void)
> >       if (cached)
> >               return cached_result;
> >
> > -     if (sysfs__read_int("devices/system/cpu/smt/active", &cached_result) > 0)
> > -             goto done;
> > +     if (sysfs__read_int("devices/system/cpu/smt/active", &cached_result) >= 0) {
> > +             cached = true;
> > +             return cached_result;
> > +     }
> >
> > +     cached_result = 0;
> >       ncpu = sysconf(_SC_NPROCESSORS_CONF);
> >       for (cpu = 0; cpu < ncpu; cpu++) {
> >               unsigned long long siblings;
> > @@ -35,18 +88,13 @@ int smt_on(void)
> >                               continue;
> >               }
> >               /* Entry is hex, but does not have 0x, so need custom parser */
> > -             siblings = strtoull(str, NULL, 16);
> > +             siblings = hweight_str(str);
> >               free(str);
> > -             if (hweight64(siblings) > 1) {
> > +             if (siblings > 1) {
> >                       cached_result = 1;
> > -                     cached = true;
> >                       break;
> >               }
> >       }
> > -     if (!cached) {
> > -             cached_result = 0;
> > -done:
> > -             cached = true;
> > -     }
> > +     cached = true;
> >       return cached_result;
> >  }
> > --
> > 2.34.0.rc2.393.gf8c9666880-goog
>
> --
>
> - Arnaldo