linux-kernel - Re: [PATCH] perf/x86/intel: Add a quirk for the calculation of the number of counters on Alder Lake

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Date:   Wed, 12 Jan 2022 10:54:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        damarion@...co.com, edison_chan_gz@...mail.com,
        ray.kinsella@...el.com, stable@...r.kernel.org
Subject: Re: [PATCH] perf/x86/intel: Add a quirk for the calculation of the
 number of counters on Alder Lake

On Tue, Jan 11, 2022 at 10:20:38AM -0800, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> For some Alder Lake machine with all E-cores disabled in a BIOS, the
> below warning may be triggered.
> 
> [ 2.010766] hw perf events fixed 5 > max(4), clipping!
> 
> Current perf code relies on the CPUID leaf 0xA and leaf 7.EDX[15] to
> calculate the number of the counters and follow the below assumption.
> 
> For a hybrid configuration, the leaf 7.EDX[15] (X86_FEATURE_HYBRID_CPU)
> is set. The leaf 0xA only enumerate the common counters. Linux perf has
> to manually add the extra GP counters and fixed counters for P-cores.
> For a non-hybrid configuration, the X86_FEATURE_HYBRID_CPU should not
> be set. The leaf 0xA enumerates all counters.
> 
> However, that's not the case when all E-cores are disabled in a BIOS.
> Although there are only P-cores in the system, the leaf 7.EDX[15]
> (X86_FEATURE_HYBRID_CPU) is still set. But the leaf 0xA is updated
> to enumerate all counters of P-cores. The inconsistency triggers the
> warning.
> 
> Several software ways were considered to handle the inconsistency.
> - Drop the leaf 0xA and leaf 7.EDX[15] CPUID enumeration support.
>   Hardcode the number of counters. This solution may be a problem for
>   virtualization. A hypervisor cannot control the number of counters
>   in a Linux guest via changing the guest CPUID enumeration anymore.
> - Find another CPUID bit that is also updated with E-cores disabled.
>   There may be a problem in the virtualization environment too. Because
>   a hypervisor may disable the feature/CPUID bit.
> - The P-cores have a maximum of 8 GP counters and 4 fixed counters on
>   ADL. The maximum number can be used to detect the case.
>   This solution is implemented in this patch.

ARGH!! This is horrific :-(

This is also the N-th problem with hybrid enumeration; is there a plan
to fix all that for the next generation or are we going to keep muddling
things?

> Fixes: ee72a94ea4a6 ("perf/x86/intel: Fix fixed counter check warning for some Alder Lake")
> Reported-by: Damjan Marion (damarion) <damarion@...co.com>
> Tested-by: Damjan Marion (damarion) <damarion@...co.com>
> Reported-by: Chan Edison <edison_chan_gz@...mail.com>
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> Cc: stable@...r.kernel.org
> ---
>  arch/x86/events/intel/core.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 187906e..f1201e8 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -6239,6 +6239,18 @@ __init int intel_pmu_init(void)
>  			pmu->num_counters = x86_pmu.num_counters;
>  			pmu->num_counters_fixed = x86_pmu.num_counters_fixed;
>  		}
> +
> +		/* Quirk: For some Alder Lake machine, when all E-cores are disabled in
> +		 * a BIOS, the leaf 0xA will enumerate all counters of P-cores. However,
> +		 * the X86_FEATURE_HYBRID_CPU is still set. The above codes will
> +		 * mistakenly add extra counters for P-cores. Correct the number of
> +		 * counters here.
> +		 */

I fixed that comment style for you.

> +		if ((pmu->num_counters > 8) || (pmu->num_counters_fixed > 4)) {
> +			pmu->num_counters = x86_pmu.num_counters;
> +			pmu->num_counters_fixed = x86_pmu.num_counters_fixed;
> +		}
> +
>  		pmu->max_pebs_events = min_t(unsigned, MAX_PEBS_EVENTS, pmu->num_counters);
>  		pmu->unconstrained = (struct event_constraint)
>  					__EVENT_CONSTRAINT(0, (1ULL << pmu->num_counters) - 1,
> -- 
> 2.7.4
>