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

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Jan 12, 2026, at 9:03 AM, Joel Fernandes <joelagnelf@nvidia.com> wrote:
> 
> 
> 
>> On Jan 12, 2026, at 4:44 AM, Vishal Chourasia <vishalc@linux.ibm.com> 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.

Also, why not just use the expedite api at the callsite that is slow than blanket expediting everything between hotplug lock and unlock.  That is more specific fix than this fix which applies more broadly to all operations. It appears the report you provided does provide the culprit callsite.

 - Joel

 


> 
> thanks,
> 
> - Joel
> 
>> 
>> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b
>> 
>> [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com
>> 
>> Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>
>> ---
>> 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
>>