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

[Vishal Chourasia <vishalc@linux.ibm.com>]

On 12/01/26 17:51, Shrikanth Hegde wrote:
>
>
> On 1/12/26 3:13 PM, Vishal Chourasia 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.
>>
>> 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
>>
>> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b
>>
>
> Hi Vishal,
>
> I tried on tip/master at 315f416d3e26.
> It fails to apply. is rcu tree updated?
I'm currently working off the GitHub mirror (|github.com/torvalds/linux)|
>
>
>> [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);
>> +
>
> Why extern is needed? All it needs is declarations no?
Already declared in kernel/rcu/rcu.h

kernel/cpu.c already includes linux/rcupdate.h, therefore
added an extern.

>
>
>>   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)
>
> Have you tested kexec path or suspend/resume path?
I did test kexec patch by booting into another kernel via kexec path.
But, I didn't test suspend/resume.

> Seems like the counter can nest, but would be good to verify.