linux-kernel - Re: [RFC][PATCH v2 08/11] context_tracking,rcu: Replace RCU dynticks counter with context

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Message-ID: <20210929191115.GY4323@worktop.programming.kicks-ass.net>
Date:   Wed, 29 Sep 2021 21:11:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org,
        mbenes@...e.cz, pmladek@...e.com, mingo@...nel.org,
        linux-kernel@...r.kernel.org, joe.lawrence@...hat.com,
        fweisbec@...il.com, tglx@...utronix.de, hca@...ux.ibm.com,
        svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, rostedt@...dmis.org, x86@...nel.org
Subject: Re: [RFC][PATCH v2 08/11] context_tracking,rcu: Replace RCU dynticks
 counter with context_tracking

On Wed, Sep 29, 2021 at 11:37:01AM -0700, Paul E. McKenney wrote:

> > +void context_tracking_idle(void)
> > +{
> > +	atomic_add_return(CT_SEQ, &raw_cpu_ptr(&context_tracking)->seq);
> 
> This is presumably a momentary idle.

> >  notrace void rcu_momentary_dyntick_idle(void)
> >  {
> > -	int seq;
> > -
> >  	raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
> > -	seq = rcu_dynticks_inc(2);
> > -	/* It is illegal to call this from idle state. */
> > -	WARN_ON_ONCE(!(seq & 0x1));
> > +	context_tracking_idle();
> >  	rcu_preempt_deferred_qs(current);
> >  }

It's whatever that is. It increments the actual sequence count without
modifying the state.