Re: [PATCH v5 1/1] cpuhp: Expedite RCU when toggling system-wide SMT mode

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Nov 12, 2026, at 3:30 AM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Mon, Jun 15, 2026 at 09:55:16AM +0530, 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 an 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.
>> 
>> Tested-by: Samir <samir@linux.ibm.com>
>> Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>> Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>
> 
> From an RCU viewpoint:
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> 
> I am guessing that you will be taking this up some other path, but if
> you would like it to to via RCU, please let me know.

I do feel there is a strong case, as the change is simple and the performance issue is kind of ridiculous, and the authors have done due diligence. 

Since Paul has provided his tag and I am helping with 7.5 merge window for RCU, I will run some tests on this (no promises) but if someone does not want me to apply it - maybe now would be a great time to yell at me.

thanks,

Joel Fernandes

> 
>                            Thanx, Paul
> 
>> ---
>> 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);
>> --
>> 2.54.0
>>