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SubjectRe: [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@linux.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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