linux-kernel - Re: [PATCH] perf/x86/intel: Clean up PEBS-via-PT on hybrid

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Message-ID: <87y0ywzj7o.fsf@ubik.fi.intel.com>
Date: Mon, 27 Jan 2025 11:42:19 +0200
From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To: kan.liang@...ux.intel.com, peterz@...radead.org, mingo@...hat.com,
 acme@...nel.org, namhyung@...nel.org, irogers@...gle.com,
 adrian.hunter@...el.com, linux-kernel@...r.kernel.org,
 alexander.shishkin@...ux.intel.com
Cc: Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [PATCH] perf/x86/intel: Clean up PEBS-via-PT on hybrid

[sending again, because smtp is hard, for some reason]
kan.liang@...ux.intel.com writes:

> From: Kan Liang <kan.liang@...ux.intel.com>
>
> The PEBS-via-PT feature is exposed for the e-core of some hybrid
> platforms, e.g., ADL and MTL. But it never works.
>
> $ dmesg | grep PEBS
> [    1.793888] core: cpu_atom PMU driver: PEBS-via-PT
>
> $ perf record -c 1000 -e '{intel_pt/branch=0/,
> cpu_atom/cpu-cycles,aux-output/pp}' -C8
> Error:
> The sys_perf_event_open() syscall returned with 22 (Invalid argument)
> for event (cpu_atom/cpu-cycles,aux-output/pp).
> "dmesg | grep -i perf" may provide additional information.
>
> The "PEBS-via-PT" is printed if the corresponding bit of per-PMU
> capabilities is set. Since the feature is supported by the e-core HW,
> perf sets the bit for e-core. However, for Intel PT, if a feature is not
> supported on all CPUs, it is not supported at all. The PEBS-via-PT event
> cannot be created successfully.
>
> The PEBS-via-PT is no longer enumerated on the latest hybrid platform. It
> will be deprecated on future platforms with Arch PEBS. Let's remove it
> from the existing hybrid platforms.

There are still lots of them in use in the wild, I'd still like to fix
this properly instead of chopping it off. Unless there's a reason not to?

Thanks,
--
Alex