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

[Joel Fernandes]

> On Jan 12, 2026, at 9:24 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Mon, Jan 12, 2026 at 02:20:44PM +0000, Joel Fernandes wrote:
>> 
>> 
>>>> On Jan 12, 2026, at 9:03 AM, Joel Fernandes <joelagnelf@xxxxxxxxxx> wrote:
>>> 
>>> 
>>> 
>>>> 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.
>> 
>> 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.
> 
> Because hotplug is not a fast path; there is no expectation of
> performance here.

Agreed, I was just wondering if it was incredibly slow or something. Looking forward to more justification from Vishal on usecase,

 - Joel


>