Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations

[Joel Fernandes]

> On Jan 12, 2026, at 4:44 AM, Vishal Chourasia <vishalc@xxxxxxxxxxxxx> wrote:
> 
> Bulk CPU hotplug operations—such as switching SMT modes across all
> cores—require hotplugging multiple CPUs in rapid succession. On large
> systems, this process takes significant time, increasing as the number
> of CPUs grows, leading to substantial delays on high-core-count
> machines. Analysis [1] reveals that the majority of this time is spent
> waiting for synchronize_rcu().
> 
> Expedite synchronize_rcu() during the hotplug path to accelerate the
> operation. Since CPU hotplug is a user-initiated administrative task,
> it should complete as quickly as possible.

When does the user initiate this in your system?

Hotplug should not be happening that often to begin with, it is a slow path that
depends on the disruptive stop-machine mechanism. 

> 
> Performance data on a PPC64 system with 400 CPUs:
> 
> + ppc64_cpu --smt=1 (SMT8 to SMT1)
> Before: real 1m14.792s
> After:  real 0m03.205s  # ~23x improvement
> 
> + ppc64_cpu --smt=8 (SMT1 to SMT8)
> Before: real 2m27.695s
> After:  real 0m02.510s  # ~58x improvement

This does look compelling but, Could you provide more information about how this was tested - what does the ppc binary do (how many hot plugs , how does the performance change with cycle count etc)?

Can you also run rcutorture testing? Some of the scenarios like TREE03 stress hotplug.

thanks,

- Joel

> 
> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b
> 
> [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@xxxxxxxxxxxxxxxxxx
> 
> Signed-off-by: Vishal Chourasia <vishalc@xxxxxxxxxxxxx>
> ---
> include/linux/rcupdate.h | 3 +++
> kernel/cpu.c             | 2 ++
> 2 files changed, 5 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index c5b30054cd01..03c06cfb2b6d 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1192,6 +1192,9 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
> extern int rcu_expedited;
> extern int rcu_normal;
> 
> +extern void rcu_expedite_gp(void);
> +extern void rcu_unexpedite_gp(void);
> +
> DEFINE_LOCK_GUARD_0(rcu,
>    do {
>        rcu_read_lock();
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 8df2d773fe3b..6b0d491d73f4 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -506,12 +506,14 @@ EXPORT_SYMBOL_GPL(cpus_read_unlock);
> 
> void cpus_write_lock(void)
> {
> +    rcu_expedite_gp();
>    percpu_down_write(&cpu_hotplug_lock);
> }
> 
> void cpus_write_unlock(void)
> {
>    percpu_up_write(&cpu_hotplug_lock);
> +    rcu_unexpedite_gp();
> }
> 
> void lockdep_assert_cpus_held(void)
> --
> 2.52.0
>