linux-kernel - Re: [PATCH v2] powerpc/kexec: Enable SMT before waking offline CPUs

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Message-ID: <knhs5gainynhozku6kb2ygxy63gyy73sbnqg3vcizk45oatzry@uig2z3mlux5u>
Date: Wed, 29 Oct 2025 09:06:38 +0530
From: "Nysal Jan K.A." <nysal@...ux.ibm.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Srikar Dronamraju <srikar@...ux.ibm.com>,
        Sachin P Bappalige <sachinpb@...ux.ibm.com>, stable@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH v2] powerpc/kexec: Enable SMT before waking offline CPUs

hi Shrikanth,

On Tue, Oct 28, 2025 at 10:56:05PM +0530, Shrikanth Hegde wrote:
> Hi Nysal.
> 
> On 10/28/25 4:25 PM, Nysal Jan K.A. wrote:

[snip]

> > --- a/arch/powerpc/kexec/core_64.c
> > +++ b/arch/powerpc/kexec/core_64.c
> > @@ -202,6 +202,23 @@ static void kexec_prepare_cpus_wait(int wait_state)
> >   	mb();
> >   }
> > +
> > +/*
> > + * The add_cpu() call in wake_offline_cpus() can fail as cpu_bootable()
> > + * returns false for CPUs that fail the cpu_smt_thread_allowed() check
> > + * or non primary threads if SMT is disabled. Re-enable SMT and set the
> > + * number of SMT threads to threads per core.
> > + */
> > +static void kexec_smt_reenable(void)
> > +{
> > +#if defined(CONFIG_SMP) && defined(CONFIG_HOTPLUG_SMT)
> > +	lock_device_hotplug();
> 
> I was looking at usage of lock_device_hotplug, looks like a good candidate for
> guard() use case. Could be done on its own patch/series.
> 

Agree, we can look at it as a separate patch.

> > +	cpu_smt_num_threads = threads_per_core;
> > +	cpu_smt_control = CPU_SMT_ENABLED;
> > +	unlock_device_hotplug();
> > +#endif
> > +}
> 
> 
> Will this work too? It might be better since we anyway going to bring that CPU up
> by doing add_cpu afterwords.
> 
> 	cpu_smt_num_threads = threads_per_core;
> 	cpuhp_smt_enable()
> 

There is some reasoning in 4d37cc2dc3df, which made the switch to use the core
device API, against calling cpu_up() directly. The other issue is 
cpuhp_smt_enable() can skip bringing up a CPU in certain cases, for example
when a core is offline.

> > +
> >   /*
> >    * We need to make sure each present CPU is online.  The next kernel will scan
> >    * the device tree and assume primary threads are online and query secondary
> > @@ -216,6 +233,8 @@ static void wake_offline_cpus(void)
> >   {
> >   	int cpu = 0;
> > +	kexec_smt_reenable();
> > +
> 
> If we do above, just change the below logic to complain if any present CPU is offline.
> 
> >   	for_each_present_cpu(cpu) {
> >   		if (!cpu_online(cpu)) {
> >   			printk(KERN_INFO "kexec: Waking offline cpu %d.\n",
> 

Thanks for the review.

--Nysal