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

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Date:	Wed, 25 Feb 2009 17:26:45 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 05:00:24PM +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).

 
> 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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