linux-kernel - Re: RCU vs NOHZ

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Date:   Thu, 29 Sep 2022 18:23:16 +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 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.

nohz_full is a gimmick that shouldn't be used outside of very specific
cases. Regular nohz otoh is used by everybody always.