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

[Samir M]

On 12/01/26 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

[1] 
https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.ca...@linux.vnet.ibm.com

Signed-off-by: Vishal Chourasia <vish...@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)
Hi Vishal,

I verified this patch using the configuration described below.
Configuration:
    •    Kernel version: 6.19.0-rc5
    •    Number of CPUs: 2048


Using this setup, I evaluated the patch with both SMT enabled and SMT 
disabled. patch shows a significant improvement in the SMT=off case and 
a measurable improvement in the SMT=on case.
The results indicate that when SMT is enabled, the system time is 
noticeably higher. In contrast, with SMT disabled, no significant 
increase in system time is observed.

SMT=ON  -> sys 31m18.849s
SMT=OFF -> sys 0m0.087s


SMT Mode    | Without Patch    | With Patch | % Improvement    |
------------------------------------------------------------------
SMT=off     | 30m 53.194s      |  6m 4.250s  | +80.40%          |
SMT=on      | 49m 5.920s       | 36m 50.386s | +25.01%          |


Please add below tag: 
Tested-by: Samir M <sa...@linux.ibm.com>

Regards,
Samir