linux-kernel - Re: [PATCH v2] x86/tsc: Extend watchdog check exemption to 4-Sockets platform

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Date:   Fri, 14 Oct 2022 08:37:18 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra" <peterz@...radead.org>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <rui.zhang@...el.com>,
        <tim.c.chen@...el.com>, Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Yu Liao <liaoyu15@...wei.com>
Subject: Re: [PATCH v2] x86/tsc: Extend watchdog check exemption to 4-Sockets
 platform

On Thu, Oct 13, 2022 at 09:02:43AM -0700, Dave Hansen wrote:
> On 10/13/22 06:12, Feng Tang wrote:
> > @@ -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 <= 4)
> >  		tsc_disable_clocksource_watchdog();
> 
> I still don't think we should perpetuate this hack.
> 
> This just plain doesn't work in numa=off numa=fake=... or presumably in
> cases where NUMA is disabled in the firmware and memory is interleaved
> across all sockets.
> 
> It also presumably doesn't work on two-socket systems that have
> Cluster-on-Die or Sub-NUMA-Clustering where a single socket is chopped
> up into multiple nodes.

Yes, after you raised the 'nr_online_nodes' issue, Peter, Rui and I
have discussed the problem, and plan to post a RFC patch as in 
 https://lore.kernel.org/lkml/Y0UgeUIJSFNR4mQB@feng-clx/

Which can cover:
 - numa=fake=... case
 - platform has DRAM nodes and cpu-less HBM/PMEM nodes

and 'sub-numa-clustering' can't be covered, and the tsc will be
watchdoged as before.

For numa=off case, there is only one CPU up, and I think lifting this
watchdog for tsc is fine.

Thanks,
Feng