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

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Message-ID: <20260112142350.GM830755@noisy.programming.kicks-ass.net>
Date: Mon, 12 Jan 2026 15:23:50 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Vishal Chourasia <vishalc@...ux.ibm.com>,
	"rcu@...r.kernel.org" <rcu@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"paulmck@...nel.org" <paulmck@...nel.org>,
	"frederic@...nel.org" <frederic@...nel.org>,
	"neeraj.upadhyay@...nel.org" <neeraj.upadhyay@...nel.org>,
	"josh@...htriplett.org" <josh@...htriplett.org>,
	"boqun.feng@...il.com" <boqun.feng@...il.com>,
	"urezki@...il.com" <urezki@...il.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"sshegde@...ux.ibm.com" <sshegde@...ux.ibm.com>,
	"srikar@...ux.ibm.com" <srikar@...ux.ibm.com>
Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug
 operations

On Mon, Jan 12, 2026 at 02:20:44PM +0000, Joel Fernandes wrote:
> 
> 
> > On Jan 12, 2026, at 9:03 AM, Joel Fernandes <joelagnelf@...dia.com> wrote:
> > 
> > 
> > 
> >> On Jan 12, 2026, at 4:44 AM, Vishal Chourasia <vishalc@...ux.ibm.com> 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.
> > 
> > When does the user initiate this in your system?
> > 
> > Hotplug should not be happening that often to begin with, it is a slow path that
> > depends on the disruptive stop-machine mechanism.
> > 
> >> 
> >> 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
> > 
> > This does look compelling but, Could you provide more information about how this was tested - what does the ppc binary do (how many hot plugs , how does the performance change with cycle count etc)?
> > 
> > Can you also run rcutorture testing? Some of the scenarios like TREE03 stress hotplug.
> 
> Also, why not just use the expedite api at the callsite that is slow
> than blanket expediting everything between hotplug lock and unlock.
> That is more specific fix than this fix which applies more broadly to
> all operations. It appears the report you provided does provide the
> culprit callsite.

Because hotplug is not a fast path; there is no expectation of
performance here.