linux-kernel - Re: [PATCH v3 0/2] x86/smpboot: tidy sched-topology and drop useless SMT level

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Message-ID: <4e2afb04-7317-4689-95c0-dfa82311f256@amd.com>
Date: Wed, 25 Jun 2025 11:20:39 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Li Chen <me@...ux.beauty>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H . Peter Anvin" <hpa@...or.com>,
 "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
 Peter Zijlstra <peterz@...radead.org>, Sohil Mehta <sohil.mehta@...el.com>,
 Brian Gerst <brgerst@...il.com>,
 Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] x86/smpboot: tidy sched-topology and drop useless
 SMT level

Hello Li,

On 6/25/2025 9:15 AM, Li Chen wrote:
> From: Li Chen <chenl311@...natelecom.cn>
> 
> This two–patch series cleans up sched-domain topology handling and
> eliminates hundreds of pointless attach/destroy cycles for large
> machines when SMT is not available.
> 
> Patch 1 (from Thomas, unchanged) gets rid of the #ifdef maze in
> build_sched_topology() by statically initialising the topology array.
> 
> Patch 2 (mine) is a follow-up that simply memmoves the array when
> cpu_smt_num_threads <= 1, so the SMT level never gets created and
> immediately torn down again.
> 
> Tested on Qemu.

I think there is a simpler approach to Patch 2 but other than that
I gave this series a spin during my review with different topology
on QEMU and didn't find any surprises. I tested with and without
the "threads" parameter to "-smp" and also with "nosmt" kernel
cmdline and everything seemed to be in order. Feel free to add:

Tested-by: K Prateek Nayak <kprateek.nayak@....com>

> 
> changelog:
> v2: fix wording issue as suggested by Thomas [1]
> v3: remove pointless memset and adjust PKG index accordingly, as
>      suggested by Thomas [2], and refine some other wording issues.
> 
> [1]: https://lore.kernel.org/all/87msa2r018.ffs@tglx/
> [2]: https://lore.kernel.org/all/875xglntx1.ffs@tglx/
> 
> Li Chen (1):
>    x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled
> 
> Thomas Gleixner (1):
>    x86/smpboot: Decrapify build_sched_topology()
> 
>   arch/x86/kernel/smpboot.c | 59 +++++++++++++++++++++------------------
>   1 file changed, 32 insertions(+), 27 deletions(-)
> 

-- 
Thanks and Regards,
Prateek