Re: [PATCH v3 2/2] x86/smpboot: avoid SMT domain attach/destroy if SMT is not enabled

[Li Chen <me@linux.beauty>]

Hi K,

 ---- On Wed, 25 Jun 2025 13:45:22 +0800  K Prateek Nayak <kprateek.nayak@amd.com> wrote --- 
 > Hello Li,
 > 
 > On 6/25/2025 9:15 AM, Li Chen wrote:
 > > From: Li Chen <chenl311@chinatelecom.cn>
 > > 
 > > Currently, the SMT domain is added into sched_domain_topology by default.
 > > 
 > > If cpu_attach_domain() finds that the CPU SMT domain’s cpumask_weight
 > > is just 1, it will destroy it.
 > > 
 > > On a large machine, such as one with 512 cores, this results in
 > > 512 redundant domain attach/destroy operations.
 > > 
 > > Avoid these unnecessary operations by simply checking
 > > cpu_smt_num_threads and remove SMT domain if the SMT domain is not
 > > enabled, and adjust the PKG index accordingly if NUMA-in-package
 > > invalidates that level as well.
 > > 
 > > Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
 > > ---
 > > changelog:
 > > v2: fix wording issue as suggested by Thomas [1]
 > > v3: remove pointless memset and adjust PKG index accordingly,
 > >      as suggested by Thomas [2]
 > > 
 > > [1]: https://lore.kernel.org/all/87msa2r018.ffs@tglx/
 > > [2]: https://lore.kernel.org/all/875xglntx1.ffs@tglx/
 > > 
 > >   arch/x86/kernel/smpboot.c | 18 +++++++++++++++++-
 > >   1 file changed, 17 insertions(+), 1 deletion(-)
 > > 
 > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
 > > index 7d202f9785362..4b6daa1545445 100644
 > > --- a/arch/x86/kernel/smpboot.c
 > > +++ b/arch/x86/kernel/smpboot.c
 > > @@ -494,13 +494,29 @@ static struct sched_domain_topology_level x86_topology[] = {
 > >   
 > >   static void __init build_sched_topology(void)
 > >   {
 > > +    bool smt_dropped = false;
 > > +
 > > +    if (cpu_smt_num_threads <= 1) {
 > > +        /*
 > > +         * SMT level is x86_topology[0].  Shift the array left by one,
 > > +         */
 > > +        memmove(&x86_topology[0], &x86_topology[1],
 > > +            sizeof(x86_topology) - sizeof(x86_topology[0]));
 > > +        smt_dropped = true;
 > > +    }
 > > +
 > 
 > Instead of this memmove, wouldn't just passing the "&x86_topology[1]"
 > to set_sched_topology() when "cpu_smt_num_threads <= 1"  work?
 > 
 > Something like below:
 > 
 > (Only tested on a QEMU based VM)
 > 
 > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
 > index 3818b16c19e1..1793248d28cd 100644
 > --- a/arch/x86/kernel/smpboot.c
 > +++ b/arch/x86/kernel/smpboot.c
 > @@ -494,6 +494,8 @@ static struct sched_domain_topology_level x86_topology[] = {
 >   
 >   static void __init build_sched_topology(void)
 >   {
 > +    struct sched_domain_topology_level *topology = x86_topology;
 > +
 >       /*
 >        * When there is NUMA topology inside the package invalidate the
 >        * PKG domain since the NUMA domains will auto-magically create the
 > @@ -504,7 +506,15 @@ static void __init build_sched_topology(void)
 >   
 >           memset(&x86_topology[pkgdom], 0, sizeof(x86_topology[pkgdom]));
 >       }
 > -    set_sched_topology(x86_topology);
 > +
 > +    /*
 > +     * Drop the SMT domains if there is only one thread per-core
 > +     * since it'll get degenerated by the scheduler anyways.
 > +     */
 > +    if (cpu_smt_num_threads <= 1)
 > +        ++topology;
 > +
 > +    set_sched_topology(topology);
 >   }
 >   
 >   void set_cpu_sibling_map(int cpu)
 
Yes, I also agree. I would switch to it in the next series and add your signoff.
Thanks!

 > >       /*
 > >        * When there is NUMA topology inside the package invalidate the
 > >        * PKG domain since the NUMA domains will auto-magically create the
 > >        * right spanning domains based on the SLIT.
 > >        */
 > >       if (x86_has_numa_in_package) {
 > > -        unsigned int pkgdom = ARRAY_SIZE(x86_topology) - 2;
 > > +        unsigned int pkgdom;
 > > +
 > > +        if (smt_dropped)
 > > +            pkgdom = ARRAY_SIZE(x86_topology) - 3;
 > > +        else
 > > +            pkgdom = ARRAY_SIZE(x86_topology) - 2;
 > >   
 > >           memset(&x86_topology[pkgdom], 0, sizeof(x86_topology[pkgdom]));
 > >       }
 > 
 > -- 
 > Thanks and Regards,
 > Prateek
 > 
 > 

Regards,
Li