linux-kernel - Re: [PATCH v2] EXP rcu: Move expedited grace period (GP) work to RT kthread

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Date:   Fri, 8 Apr 2022 08:58:34 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        kernel-team <kernel-team@...roid.com>, Tejun Heo <tj@...nel.org>,
        Tim Murray <timmurray@...gle.com>, Wei Wang <wvw@...gle.com>,
        Kyle Lin <kylelin@...gle.com>,
        Chunwei Lu <chunweilu@...gle.com>,
        Lulu Wang <luluw@...gle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        rcu <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] EXP rcu: Move expedited grace period (GP) work to RT
 kthread_worker

On Fri, Apr 08, 2022 at 11:39:46AM -0400, Steven Rostedt wrote:
> On Fri, 8 Apr 2022 06:21:03 -0400
> Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> > > +       default !PREEMPT_RT && NR_CPUS <= 32  
> > 
> > What is the benefit of turning it off on PREEMPT_RT, even if
> > PREEMPT_RT does not use expedited GPs much post-boot? I would think in
> > the future if PREEMPT_RT ever uses expedited GPs, they would want this
> > feature even more. I'd rather be future-proof now as I don't see any
> > advantages of disabling it on !PREEMPT_RT (And a drawback that the fix
> > won't apply to those systems). Also will keep the config simple.
> 
> The default kthread priority is 1. This should not affect PREEMPT_RT, as
> PREEMPT_RT users are usually more concerned by the performance of their
> higher priority tasks. Priority 1 is not considered an issue.
> 
> I do not see why PREEMPT_RT is special here. Why was it singled out?

Because I didn't see the point of doing kthread_create_worker() to create
a pair of kthread_worker structuress that were never going to be used
in PREEMPT_RT kernels.

Or are PREEMPT_RT kernels no longer booting with rcupdate.rcu_normal?
Last I knew, they all did this to avoid IPI latencies from expedited
grace periods.  Has this changed?

							Thanx, Paul