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Date:   Thu, 14 Sep 2023 14:25:40 +0530
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH] perf/x86/amd: Do not WARN on every IRQ
Content-Language: en-US
To:     Jirka Hladky <jhladky@redhat.com>, Breno Leitao <leitao@debian.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, leit@fb.com,
        dcostantino@meta.com,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@vger.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-kernel@vger.kernel.org>
References: <20230616115316.3652155-1-leitao@debian.org>
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From:   Sandipan Das <sandipan.das@amd.com>
In-Reply-To: <CAE4VaGBXO5Joynan_o13XpT=mNrwt4D84NTZF6H62GXJjpMbBA@mail.gmail.com>
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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.
>>
> 
>