Re: [PATCH v4 5/6] perf vendor events arm64: Update stall_slot workaround for N2 r0p3

[John Garry <john.g.garry@oracle.com>]

On 07/08/2023 15:20, James Clark wrote:

Hi James,

Thanks for the effort in doing this.

> N2 r0p3 doesn't require the workaround [1], so gating on (#slots - 5) no
> longer works because all N2s have 5 slots. Add a new expression builtin
> that identifies the need for the workaround correctly.
> 
> [1]: https://urldefense.com/v3/__https://gitlab.arm.com/telemetry-solution/telemetry-solution/-/blob/main/data/pmu/cpu/neoverse/neoverse-n2-r0p3.json__;!!ACWV5N9M2RV99hQ!Nx1xgWXXS9GBasNpOKQXHWKe8VwpRB0h8lAfOmwUmkkOQTjFqn2NswO8ti8vTcblUfAYN9NAtxqAh-sf0TEOvQ$
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>   tools/perf/arch/arm64/util/pmu.c              | 21 +++++++++++++++++++
>   .../arm64/arm/neoverse-n2-v2/metrics.json     |  8 +++----
>   tools/perf/util/expr.c                        |  4 ++++
>   tools/perf/util/pmu.c                         |  6 ++++++
>   tools/perf/util/pmu.h                         |  1 +
>   5 files changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/perf/arch/arm64/util/pmu.c b/tools/perf/arch/arm64/util/pmu.c
> index 561de0cb6b95..30e2385a83cf 100644
> --- a/tools/perf/arch/arm64/util/pmu.c
> +++ b/tools/perf/arch/arm64/util/pmu.c
> @@ -2,6 +2,7 @@
>   
>   #include <internal/cpumap.h>
>   #include "../../../util/cpumap.h"
> +#include "../../../util/header.h"
>   #include "../../../util/pmu.h"
>   #include "../../../util/pmus.h"
>   #include <api/fs/fs.h>
> @@ -62,3 +63,23 @@ double perf_pmu__cpu_slots_per_cycle(void)
>   
>   	return slots ? (double)slots : NAN;
>   }
> +
> +double perf_pmu__no_stall_errata(void)

While I like the approach of encoding the per-CPU support in the metric 
expression, I find that literal "no stall errata" is vague and could 
apply to any "stall errata" for any SoC for any architecture.

If we were going to do it this way, then we would need a more specific 
name, like perf_pmu__no_stall_errata_arm64_errata123456(), but that 
should not be in the core perf code.

Could we instead add a function to check cpuid and have something like 
this in the JSON:

"MetricExpr": "(op_retired / op_spec) * (1 - (stall_slot if 
(cpuid_less_than(410fd493)) else (stall_slot - cpu_cycles)) / (#slots * 
cpu_cycles))"

I'm currently figuring out how cpuid_less_than() would be implemented 
(I'm no great python wrangler), but it would be along the lines of what 
Ian added for "has_event" in 
https://lore.kernel.org/linux-perf-users/20230623151016.4193660-1-irogers@google.com/

Thanks,
John