linux-kernel - Re: [PATCH] cpuhp: Expedite synchronize

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Message-ID: <9e57a091-2f34-402a-972a-31dd9a10664b@linux.ibm.com>
Date: Sun, 18 Jan 2026 17:08:44 +0530
From: Samir M <samir@...ux.ibm.com>
To: Vishal Chourasia <vishalc@...ux.ibm.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc: paulmck@...nel.org, frederic@...nel.org, neeraj.upadhyay@...nel.org,
        joelagnelf@...dia.com, josh@...htriplett.org, boqun.feng@...il.com,
        urezki@...il.com, rostedt@...dmis.org, tglx@...utronix.de,
        peterz@...radead.org, sshegde@...ux.ibm.com, srikar@...ux.ibm.com
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations


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.camel@linux.vnet.ibm.com
>
> Signed-off-by: Vishal Chourasia <vishalc@...ux.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 <samir@...ux.ibm.com>

Regards,
Samir