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

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Message-ID: <20260113090153.GS830755@noisy.programming.kicks-ass.net>
Date: Tue, 13 Jan 2026 10:01:53 +0100
From: Peter Zijlstra <peterz@...radead.org>
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,
	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 11:30:52PM +0530, Vishal Chourasia wrote:
> Hello Peter,
> 
> 
> 
> On Mon, Jan 12, 2026 at 03:24:40PM +0100, Peter Zijlstra wrote:
> > 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
> > > 
> > 
> > But who cares? Its not like you'd *ever* do this, right?
> Users dynamically adjust SMT modes to optimize performance of the
> workload being run. And, yes it doesn't happen too often, but when it
> does, on machines with (>= 1920 CPUs) it takes more than 20 mins to
> finish.

Users cannot change this, it is root only.

Having to change SMT mode per workload seems quite insane; but whatever.

If you do have to put RCU hooks anywhere; I'd much rather see them in
cpuhp_smt_{en,dis}able(), such that they only affect the batch hotplug
case, rather than everything using cpus_write_lock().

Also note that there is a case to be made to optimize this batch hotplug
case; for one it makes no sense to take cpus_write_lock() over and over
and over again; if you can pull that out, just like it already lifted
cpu_maps_update_begin(), this would help.

And Joel has a point, in that it might make sense for RCU to behave
'better' under these conditions.