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Message-ID: <aff10f33-b379-6872-f180-b38f6a0a669a@intel.com>
Date:   Mon, 10 Oct 2022 07:23:10 -0700
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Subject: Re: [PATCH] x86/tsc: Extend the watchdog check exemption to 4S/8S
 machine
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To:     Feng Tang <feng.tang@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        tim.c.chen@intel.com, Xiongfeng Wang <wangxiongfeng2@huawei.com>,
        Yu Liao <liaoyu15@huawei.com>
References: <20221009051209.393377-1-feng.tang@intel.com>
 <Y0LGLGW7RSlklKyl@hirez.programming.kicks-ass.net>
 <Y0N0ENurfliW315D@feng-clx>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <Y0N0ENurfliW315D@feng-clx>
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On 10/9/22 18:23, Feng Tang wrote:
>>> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
>>> index cafacb2e58cc..b4ea79cb1d1a 100644
>>> --- a/arch/x86/kernel/tsc.c
>>> +++ b/arch/x86/kernel/tsc.c
>>> @@ -1217,7 +1217,7 @@ static void __init check_system_tsc_reliable(void)
>>>  	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
>>>  	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
>>>  	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
>>> -	    nr_online_nodes <= 2)
>>> +	    nr_online_nodes <= 8)
>> So you're saying all 8 socket systems since Broadwell (?) are TSC
>> sync'ed ?
> No, I didn't mean that. I haven't got chance to any 8 sockets
> machine, and I got a report last month that on one 8S machine,
> the TSC was judged 'unstable' by HPET as watchdog.

That's not a great check.  Think about numa=fake=4U, for instance.  Or a
single-socket system with persistent memory and high bandwidth memory.

Basically 'nr_online_nodes' is a software construct.  It's going to be
really hard to infer anything from it about what the _hardware_ is.