linux-kernel - Re: [PATCH] v4 Teach RCU that idle task is not quiscent state at boot

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Message-ID: <20090225171249.GD6797@linux.vnet.ibm.com>
Date:	Wed, 25 Feb 2009 09:12:49 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	vegard.nossum@...il.com, stable@...nel.org,
	akpm@...ux-foundation.org, penberg@...helsinki.fi
Subject: Re: [PATCH] v4 Teach RCU that idle task is not quiscent state at
	boot

On Wed, Feb 25, 2009 at 05:26:45PM +0100, Nick Piggin wrote:
> On Wed, Feb 25, 2009 at 05:00:24PM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > +/* Internal to kernel, but needed by rcupreempt.h. */
> > > +extern int rcu_idle_cpu_truthful;
> > 
> > The name sucks a bit ;-) 'truthful' is an emotionally laden 
> > statement and distracts from the technical purpose when reading 
> > it ;)
> > 
> > Same for:
> > 
> > > +extern void rcu_idle_now_means_idle(void);
> > 
> > Also, i'm wondering, is there really no way to avoid this quirk. 
> > We almost got away without it for a long time.
> 
> I wonder if you couldn't leave cpu_rq(cpu)->idle as NULL until right
> up to the point where it actually becomes the "idle" thread? This
> would make idle_cpu() more truthful for all other callers in early
> boot code too. And rcupdate shouldn't need any changes (except the
> num_online_cpus() == 1 shortcut probably remains as a nice opt).

It looked to me that the idle task initialization was moved early
to allow interrupt handlers to see a more normal environment, but I
could easily be confused here.

						Thanx, Paul

> > This one:
> > 
> > >  void rcu_check_callbacks(int cpu, int user)
> > >  {
> > >  	if (user ||
> > > -	    (idle_cpu(cpu) && !in_softirq() &&
> > > -				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> > > +	    (idle_cpu(cpu) && rcu_idle_cpu_truthful &&
> > > +	     !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
> > 
> > Is a hotpath called very often ...
> > 
> > 	Ingo
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