Re: [PATCH v2 4/7] perf topology: Add core_wide

[Namhyung Kim]

Hi Ian,

On Wed, Aug 31, 2022 at 10:50 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> It is possible to optimize metrics when all SMT threads (CPUs) on a
> core are measuring events in system wide mode. For example, TMA
> metrics defines CORE_CLKS for Sandybrdige as:
>
> if SMT is disabled:
>   CPU_CLK_UNHALTED.THREAD
> if SMT is enabled and recording on all SMT threads:
>   CPU_CLK_UNHALTED.THREAD_ANY / 2
> if SMT is enabled and not recording on all SMT threads:
>   (CPU_CLK_UNHALTED.THREAD/2)*
>   (1+CPU_CLK_UNHALTED.ONE_THREAD_ACTIVE/CPU_CLK_UNHALTED.REF_XCLK )
>
> That is two more events are necessary when not gathering counts on all
> SMT threads. To distinguish all SMT threads on a core vs system wide
> (all CPUs) call the new property core wide.  Add a core wide test that
> determines the property from user requested CPUs, the topology and
> system wide. System wide is required as other processes running on a
> SMT thread will change the counts.
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
[SNIP]
> diff --git a/tools/perf/util/smt.c b/tools/perf/util/smt.c
> index ce90c4ee4138..994e9e418227 100644
> --- a/tools/perf/util/smt.c
> +++ b/tools/perf/util/smt.c
> @@ -21,3 +21,17 @@ bool smt_on(const struct cpu_topology *topology)
>         cached = true;
>         return cached_result;
>  }
> +
> +bool core_wide(bool system_wide, const char *user_requested_cpu_list,
> +              const struct cpu_topology *topology)
> +{
> +       /* If not everything running on a core is being recorded then we can't use core_wide. */
> +       if (!system_wide)
> +               return false;

I'm not sure if it's correct.  Wouldn't it be ok if it runs on all
threads in a core
even if system wide is off?  I thought that's what the below code checks.

In fact I thought the opposite logic like

    if (system_wide)
        return true;

But it seems the code allows to have cpu_list in the system wide mode.
Then it also needs to check the user requested cpus being NULL.
(IMHO system_wide should be cleared when it has a cpu list...)

    if (system_wide && !user_requested_cpu_list)
        return true;

And if we have a target pointer, we could add this too.

    if (!target__has_cpu(target))
        return false;

Thanks,
Namhyung


> +
> +       /* Cheap case that SMT is disabled and therefore we're inherently core_wide. */
> +       if (!smt_on(topology))
> +               return true;
> +
> +       return cpu_topology__core_wide(topology, user_requested_cpu_list);
> +}
> diff --git a/tools/perf/util/smt.h b/tools/perf/util/smt.h
> index e26999c6b8d4..ae9095f2c38c 100644
> --- a/tools/perf/util/smt.h
> +++ b/tools/perf/util/smt.h
> @@ -7,4 +7,11 @@ struct cpu_topology;
>  /* Returns true if SMT (aka hyperthreading) is enabled. */
>  bool smt_on(const struct cpu_topology *topology);
>
> +/*
> + * Returns true when system wide and all SMT threads for a core are in the
> + * user_requested_cpus map.
> + */
> +bool core_wide(bool system_wide, const char *user_requested_cpu_list,
> +              const struct cpu_topology *topology);
> +
>  #endif /* __SMT_H */
> --
> 2.37.2.672.g94769d06f0-goog
>