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

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Message-ID: <aWTIDGQ1_NYeO4iU@pc636>
Date: Mon, 12 Jan 2026 11:08:12 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Vishal Chourasia <vishalc@...ux.ibm.com>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 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 Mon, Jan 12, 2026 at 03:13:33PM +0530, 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
> 
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.

--
Uladzislau Rezki