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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Message-ID: <abf04b7b-54c4-c82e-9a3b-53e97b73e90d@linux.intel.com>
Date:   Wed, 12 Jan 2022 10:26:39 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 1/12/2022 4:54 AM, Peter Zijlstra wrote:
> 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?

Yes, that's annoying. We are working on it for the future generation.
The internal validation team is also enhancing the test case to test 
different configurations.

> 
>> 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.

Ah, sorry for that. Thanks!

Kan

> 
>> +		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
>>