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

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Message-ID: <e6efda46-2f47-4a6e-8352-4794479e4a1f@linux.ibm.com>
Date: Mon, 12 Jan 2026 16:13:26 +0530
From: Vishal Chourasia <vishalc@...ux.ibm.com>
To: Uladzislau Rezki <urezki@...il.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, rostedt@...dmis.org,
        tglx@...utronix.de, peterz@...radead.org, sshegde@...ux.ibm.com,
        srikar@...ux.ibm.com, aboorvad@...ux.ibm.com
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations

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@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.

# 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

Regards,
vishalc