linux-kernel - Re: [PATCH] x86/tsc: Use topology_max

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Date: Mon, 18 Mar 2024 09:03:19 +0800
From: Feng Tang <feng.tang@...el.com>
To: Thomas Gleixner <tglx@...utronix.de>
CC: Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>, "Peter
 Zijlstra" <peterz@...radead.org>, <x86@...nel.org>, <paulmck@...nel.org>,
	<rui.zhang@...el.com>, Waiman Long <longman@...hat.com>,
	<linux-kernel@...r.kernel.org>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/tsc: Use topology_max_packages() to get package
 number

On Fri, Mar 15, 2024 at 09:51:32PM +0100, Thomas Gleixner wrote:
> On Fri, Mar 15 2024 at 10:58, Dave Hansen wrote:
> > On 3/15/24 04:26, Feng Tang wrote:
> >>         /*
> >>          * Today neither Intel nor AMD support heterogeneous systems so
> >>          * extrapolate the boot cpu's data to all packages.
> >>          */
> >>         ncpus = cpu_data(0).booted_cores * topology_max_smt_threads();
> >>         __max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
> >
> > Because Intel obviously has heterogeneous systems today.
> 
> Hybrid is a per package property. 
> 
> But neither Intel nor AMD support populating multi socket systems with
> random packages, where socket 0 has less cores than socket 1 or socket 0
> is hybrid and socket 1 is not.


Before posting the patch, I run the latest upstream kernel with your
topology refactoring patchset on one AlderLake and one MetorLake box,
and they both show the number of package is 1.

> 
> > So I'll buy that removing 'nr_online_nodes' takes NUMA out of the
> > picture (which is good), but I want to hear more about why
> > topology_max_packages() and '4' are the right things to be checking.
> >
> > I suspect the real reason '4' was picked was to give the calculation
> > some wiggle room because it's not actually all that precise.
> 
> IIRC the TSC is only guaranteed to be synchronized up to 4 sockets, but
> my memory might be wrong as usual.

Yes. I did try to increase the bar to '8' with a patch, and at that
time Peter Zijlstra mentioned there was real issue with TSC found on
some old 8 sockets machine.

Thanks,
Feng

> 
> Thanks,
> 
>         tglx