Re: [PATCH] perf/x86/amd: Do not WARN on every IRQ

[Sandipan Das <sandipan.das@amd.com>]

Hi Breno, Jirka,

On 9/14/2023 2:15 PM, Jirka Hladky wrote:
> Hi Breno,
> 
> I'm definitively voting for using WARN_ON_ONCE - in the current
> implementation, we are getting thousands of the same warnings and Call
> Traces, causing the system to become unusable.
> 
>> Anyway, please let me know whatever is your preferred way and I will submit a v2.
> @Peter Zijlstra and @Sandipan - could you please comment on the
> preferred implementation of the patch?
> 

I agree with using WARN_ON_ONCE() to make this less intrusive.

> 
> On Wed, Sep 13, 2023 at 6:24 PM Breno Leitao <leitao@debian.org> wrote:
>>
>> Hi Peter,
>>
>> On Fri, Jun 16, 2023 at 03:29:54PM +0200, Peter Zijlstra wrote:
>>> On Fri, Jun 16, 2023 at 04:53:15AM -0700, Breno Leitao wrote:
>>>> On some systems, the Performance Counter Global Status Register is
>>>> coming with reserved bits set, which causes the system to be unusable
>>>> if a simple `perf top` runs. The system hits the WARN() thousands times
>>>> while perf runs.
>>>>
>>>> WARNING: CPU: 18 PID: 20608 at arch/x86/events/amd/core.c:944 amd_pmu_v2_handle_irq+0x1be/0x2b0
>>>>
>>>> This happens because the "Performance Counter Global Status Register"
>>>> (PerfCntGlobalStatus) MSR has bit 7 set. Bit 7 should be reserved according
>>>> to the documentation (Figure 13-12 from "AMD64 Architecture Programmer’s
>>>> Manual, Volume 2: System Programming, 24593"[1]
>>>
>>> Would it then not make more sense to mask out bit7 before:
>>>
>>> +     status &= ~AMD_PMU_V2_GLOBAL_STATUS_RESERVED;
>>>       if (!status)
>>>               goto done;
>>
>> Instead of masking `status` against AMD_PMU_V2_GLOBAL_STATUS_RESERVED
>> (AMD64_NUM_COUNTERS?), I opted for using the `amd_pmu_global_cntr_mask`
>> global variable because it seems to represent what the loop below is
>> iterating over:
>>
>>         /* PMC Enable and Overflow bits for PerfCntrGlobal* registers */
>>         static u64 amd_pmu_global_cntr_mask __read_mostly;
>>
>> Also, I think we want to WARN_ON_ONCE() if we see this problem. Right
>> now, it warns at every time we call this function, which makes the
>> machine unusable, but, warning it once could be helpful to figure out
>> there is something wrong with the machine/firmware.
>>
>> Anyway, please let me know whatever is your preferred way and I will
>> submit a v2.
>>
> 
>