Re: [PATCH v4 1/1] cpuhp: Expedite RCU when toggling system-wide SMT mode

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 5/7/26 11:09 AM, Vishal Chourasia wrote:
> On large idle systems, changing system-wide SMT level using the sysfs
> control interface still takes approximately 40-55 minutes. Changing SMT
> levels is a user triggered operation generally done when systems are
> fairly idle. This causes users to be blocked from doing other subsequent
> work.
> 
> Analyzing the collected profile data during SMT level switch, showed
> that CPU hotplug machinery was blocked on synchronize_rcu() calls.
> 
> Expedite RCU grace periods for the entire duration of triggered SMT
> switch operation via the sysfs control interface. Individual CPU hotplug
> operations via the online/offline interface are not affected.
> 
> On a PPC64 system with 400 CPUs:
> 
>    SMT8 to SMT1:
>      before:  1m14s
>      after:   3.2s   (~23x faster)
> 
>    SMT1 to SMT8:
>      before:  2m27s
>      after:   2.5s   (~58x faster)
> 
> On a large config system with 1920 CPUs, completion time improves from
> ~1 hour to 5-6 minutes.
> 
> Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>
> ---
>   include/linux/rcupdate.h | 8 ++++++++
>   kernel/cpu.c             | 4 ++++
>   kernel/rcu/rcu.h         | 4 ----
>   3 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index bfa765132de8..b6bccf131f1c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1178,6 +1178,14 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
>   extern int rcu_expedited;
>   extern int rcu_normal;
>   
> +#ifdef CONFIG_TINY_RCU
> +static inline void rcu_expedite_gp(void) { }
> +static inline void rcu_unexpedite_gp(void) { }
> +#else
> +void rcu_expedite_gp(void);
> +void rcu_unexpedite_gp(void);
> +#endif
> +
>   DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
>   DECLARE_LOCK_GUARD_0_ATTRS(rcu, __acquires_shared(RCU), __releases_shared(RCU))
>   
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e..6351da9dffdc 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2658,6 +2658,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
>   	int cpu, ret = 0;
>   
>   	cpu_maps_update_begin();
> +	rcu_expedite_gp();
>   	for_each_online_cpu(cpu) {
>   		if (topology_is_primary_thread(cpu))
>   			continue;
> @@ -2687,6 +2688,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
>   	}
>   	if (!ret)
>   		cpu_smt_control = ctrlval;
> +	rcu_unexpedite_gp();
>   	cpu_maps_update_done();
>   	return ret;
>   }
> @@ -2704,6 +2706,7 @@ int cpuhp_smt_enable(void)
>   	int cpu, ret = 0;
>   
>   	cpu_maps_update_begin();
> +	rcu_expedite_gp();
>   	cpu_smt_control = CPU_SMT_ENABLED;
>   	for_each_present_cpu(cpu) {
>   		/* Skip online CPUs and CPUs on offline nodes */
> @@ -2717,6 +2720,7 @@ int cpuhp_smt_enable(void)
>   		/* See comment in cpuhp_smt_disable() */
>   		cpuhp_online_cpu_device(cpu);
>   	}
> +	rcu_unexpedite_gp();
>   	cpu_maps_update_done();
>   	return ret;
>   }
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index fa6d30ce73d1..d6a63ee60bc0 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -521,8 +521,6 @@ do {									\
>   static inline bool rcu_gp_is_normal(void) { return true; }
>   static inline bool rcu_gp_is_expedited(void) { return false; }
>   static inline bool rcu_async_should_hurry(void) { return false; }
> -static inline void rcu_expedite_gp(void) { }
> -static inline void rcu_unexpedite_gp(void) { }
>   static inline void rcu_async_hurry(void) { }
>   static inline void rcu_async_relax(void) { }
>   static inline bool rcu_cpu_online(int cpu) { return true; }
> @@ -530,8 +528,6 @@ static inline bool rcu_cpu_online(int cpu) { return true; }
>   bool rcu_gp_is_normal(void);     /* Internal RCU use. */
>   bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
>   bool rcu_async_should_hurry(void);  /* Internal RCU use. */
> -void rcu_expedite_gp(void);
> -void rcu_unexpedite_gp(void);
>   void rcu_async_hurry(void);
>   void rcu_async_relax(void);
>   void rcupdate_announce_bootup_oddness(void);


Given the context that,
- System is expected to be idle.
- It is user triggered event.

It makes sense to expedite the RCU at the cost of additional
IPI's. So

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>