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

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Message-ID:
 <DS0PR12MB6486E6599B99D08C5E46EC06A388A@DS0PR12MB6486.namprd12.prod.outlook.com>
Date: Mon, 19 Jan 2026 09:45:48 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Vishal Chourasia <vishalc@...ux.ibm.com>,
	boqun.feng@...il.com,
	frederic@...nel.org,
	josh@...htriplett.org,
	linux-kernel@...r.kernel.org,
	neeraj.upadhyay@...nel.org,
	paulmck@...nel.org,
	rcu@...r.kernel.org,
	rostedt@...dmis.org,
	srikar@...ux.ibm.com,
	tglx@...utronix.de,
	urezki@...il.com,
	samir@...ux.ibm.com
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during SMT switch

On Sun, Jan 19, 2026 at 03:11:18PM +0100, Peter Zijlstra wrote:
> By holding an extra rcu_sync reference, the percpu rwsem is kept into
> the the slow path, avoiding the rcu-sync on down_write(), which was very
> prevalent per this:
>
>   https://lkml.kernel.org/r/aWU9HRcs4ghazIRg@linux.ibm.com

Makes sense, though I wonder if we should have a separate percpu-rwsem API
for this rather than directly accessing the lock-internal rcu_sync state?
Other future percpu_rw_semaphore users may benefit as well.

 - Joel