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

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Date: Fri, 15 Mar 2024 21:51:32 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Dave Hansen <dave.hansen@...el.com>, Feng Tang <feng.tang@...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
Cc: 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 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.

> 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.

Thanks,

        tglx