linux-kernel - Re: RCU vs NOHZ

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Date:   Thu, 29 Sep 2022 21:01:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: RCU vs NOHZ

On Thu, Sep 29, 2022 at 09:42:04AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 29, 2022 at 06:23:16PM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 29, 2022 at 08:46:18AM -0700, Paul E. McKenney wrote:
> > > On Thu, Sep 29, 2022 at 08:20:44AM -0700, Paul E. McKenney wrote:
> > 
> > > > > > There is a directly invoked RCU hook for any transition that enables or
> > > > > > disables the tick, namely the ct_*_enter() and ct_*_exit() functions,
> > > > > > that is, those functions formerly known as rcu_*_enter() and rcu_*_exit().
> > > > > 
> > > > > Context tracking doesn't know about NOHZ, therefore RCU can't either.
> > > > > Context tracking knows about IDLE, but not all IDLE is NOHZ-IDLE.
> > > > > 
> > > > > Specifically we have:
> > > > > 
> > > > > 	ct_{idle,irq,nmi,user,kernel}_enter()
> > > > > 
> > > > > And none of them are related to NOHZ in the slightest. So no, RCU does
> > > > > not have a NOHZ callback.
> > > > > 
> > > > > I'm still thikning you're conflating NOHZ_FULL (stopping the tick when
> > > > > in userspace) and regular NOHZ (stopping the tick when idle).
> > > 
> > > Exactly how are ct_user_enter() and ct_user_exit() completely unrelated
> > > to nohz_full CPUs?
> > 
> > That's the thing; I'm not talking about nohz_full. I'm talking about
> > regular nohz. World of difference there.
> 
> And indeed, for !nohz_full CPUs, the tick continues throughout userspace
> execution.  But you really did have ct_user_enter() and ct_user_exit()
> on your list.
> 
> And for idle (as opposed to nohz_full userspace execution), there is still
> ct_{idle,irq,nmi}_enter().  And RCU does pay attention to these.
> 
> So exactly what are you trying to tell me here?  ;-)

That RCU doens't have a nohz callback -- you were arguing it does
through the ct_*_enter() things, I said none of them are related to
nohz.