linux-kernel - Re: [PATCH 6/7] perf/x86: Replace magic numbers with macros for attr

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Message-ID: <24bc287a-a3e0-45f6-a8f1-20a34e2f9af2@linux.intel.com>
Date: Thu, 20 Nov 2025 15:30:54 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Ian Rogers <irogers@...gle.com>, Falcon Thomas <thomas.falcon@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Andi Kleen <ak@...ux.intel.com>, Eranian Stephane <eranian@...gle.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 Dapeng Mi <dapeng1.mi@...el.com>, Zide Chen <zide.chen@...el.com>,
 Xudong Hao <xudong.hao@...el.com>
Subject: Re: [PATCH 6/7] perf/x86: Replace magic numbers with macros for
 attr_rdpmc


On 11/20/2025 2:19 PM, Ian Rogers wrote:
> On Wed, Nov 19, 2025 at 9:37 PM Dapeng Mi <dapeng1.mi@...ux.intel.com> wrote:
>> Use macros to replace these attr_rdpmc magic numbers, so users are easy
>> to know their meaning.
>>
>> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> I'm reminded that we were having issues with rdpmc on hybrid:
> https://lore.kernel.org/lkml/20250614004528.1652860-1-irogers@google.com/
> like the enable/disable rdpmc flag being shared across the cpu_core
> and cpu_atom PMUs, and needing to force the thread doing the rdpmc to
> have affinity matching the CPUs of the PMU it is reading from, as
> otherwise things like struct perf_event_mmap_page's index could do
> interesting things:
> https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/mmap-basic.c?h=perf-tools-next#n208
> Others required the rdpmc to be in a restartable sequence and Peter
> proposed fixing this in the kernel:
> https://lore.kernel.org/linux-perf-users/20250618084522.GE1613376@noisy.programming.kicks-ass.net/
>
> Also looks like we never merged fixing the documentation:
> https://lore.kernel.org/lkml/20220817174909.877139-1-irogers@google.com/
>
> Not specifically a DMR issue but this is reminding me of a bunch of
> tech debt - I also wonder if this patch requires the test being
> updated.

Thanks for reminding. This patch doesn't involve any functional change for
the "rdpmc" attribute, but along with the introduction of per-counter
"rdpmc user disable" in patch 7/7, the rdpmc test can definitely be
enhanced. I would enhance the rpdmc test in next version. 


>
> Thanks,
> Ian
>
>> ---
>>  arch/x86/events/core.c       | 7 ++++---
>>  arch/x86/events/intel/p6.c   | 2 +-
>>  arch/x86/events/perf_event.h | 7 +++++++
>>  3 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 5d0d5e466c62..3d9cc1d7fcfa 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -2129,7 +2129,8 @@ static int __init init_hw_perf_events(void)
>>
>>         pr_cont("%s PMU driver.\n", x86_pmu.name);
>>
>> -       x86_pmu.attr_rdpmc = 1; /* enable userspace RDPMC usage by default */
>> +       /* enable userspace RDPMC usage by default */
>> +       x86_pmu.attr_rdpmc = X86_USER_RDPMC_CONDITIONAL_ENABLE;
>>
>>         for (quirk = x86_pmu.quirks; quirk; quirk = quirk->next)
>>                 quirk->func();
>> @@ -2609,12 +2610,12 @@ static ssize_t set_attr_rdpmc(struct device *cdev,
>>                  */
>>                 if (val == 0)
>>                         static_branch_inc(&rdpmc_never_available_key);
>> -               else if (x86_pmu.attr_rdpmc == 0)
>> +               else if (x86_pmu.attr_rdpmc == X86_USER_RDPMC_NEVER_ENABLE)
>>                         static_branch_dec(&rdpmc_never_available_key);
>>
>>                 if (val == 2)
>>                         static_branch_inc(&rdpmc_always_available_key);
>> -               else if (x86_pmu.attr_rdpmc == 2)
>> +               else if (x86_pmu.attr_rdpmc == X86_USER_RDPMC_ALWAYS_ENABLE)
>>                         static_branch_dec(&rdpmc_always_available_key);
>>
>>                 on_each_cpu(cr4_update_pce, NULL, 1);
>> diff --git a/arch/x86/events/intel/p6.c b/arch/x86/events/intel/p6.c
>> index 6e41de355bd8..fb991e0ac614 100644
>> --- a/arch/x86/events/intel/p6.c
>> +++ b/arch/x86/events/intel/p6.c
>> @@ -243,7 +243,7 @@ static __init void p6_pmu_rdpmc_quirk(void)
>>                  */
>>                 pr_warn("Userspace RDPMC support disabled due to a CPU erratum\n");
>>                 x86_pmu.attr_rdpmc_broken = 1;
>> -               x86_pmu.attr_rdpmc = 0;
>> +               x86_pmu.attr_rdpmc = X86_USER_RDPMC_NEVER_ENABLE;
>>         }
>>  }
>>
>> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
>> index f7caabc5d487..24a81d2916e9 100644
>> --- a/arch/x86/events/perf_event.h
>> +++ b/arch/x86/events/perf_event.h
>> @@ -187,6 +187,13 @@ struct amd_nb {
>>          (1ULL << PERF_REG_X86_R14)   | \
>>          (1ULL << PERF_REG_X86_R15))
>>
>> +/* user space rdpmc control values */
>> +enum {
>> +       X86_USER_RDPMC_NEVER_ENABLE             = 0,
>> +       X86_USER_RDPMC_CONDITIONAL_ENABLE       = 1,
>> +       X86_USER_RDPMC_ALWAYS_ENABLE            = 2,
>> +};
>> +
>>  /*
>>   * Per register state.
>>   */
>> --
>> 2.34.1
>>