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

[Uladzislau Rezki]

Hello, Vishalc!

> Hi Uladzislau,
> 
> On 12/01/26 15:38, Uladzislau Rezki wrote:
> > On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote:
> > > 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
> > > 
> > > [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@xxxxxxxxxxxxxxxxxx
> > > 
> > Also you can try: echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> > to speedup regular synchronize_rcu() call. But i am not saying that it would beat
> > your "expedited switch" improvement.
> 
> # echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> 
> After setting,
> 
> # time ppc64_cpu --smt=1;
> real    1m10.726s          # Run 1
> real    1m12.530s          # Run 2
> 
> # time ppc64_cpu --smt=8
> real    0m36.661s          # Run 1
> real    0m41.401s          # Run 2
> 
Thanks.

"ppc64_cpu --smt=1" is the same, i assume it is offlining.
"ppc64_cpu --smt=8", whereas, onlining, sees the differences(~5x).

But your real "0m02.510s" is hard to beat event by activating the
"rcu_normal_wake_from_gp" option.

--
Uladzislau Rezki