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SubjectRe: [PATCH 2/4] context_tracking: Inherit TIF_NOHZ through forks instead of context switches
On Tue, Apr 21, 2015 at 05:26:00PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 21, 2015 at 02:23:07PM +0200, Frederic Weisbecker wrote:
> > +void __init context_tracking_cpu_set(int cpu)
> > {
> > + static __initdata bool initialized = false;
> > + struct task_struct *p, *t;
> > +
> > + if (!per_cpu(context_tracking.active, cpu)) {
> > + per_cpu(context_tracking.active, cpu) = true;
> > + static_key_slow_inc(&context_tracking_enabled);
> > + }
> > +
> > + if (initialized)
> > + return;
> > +
> > + set_tsk_thread_flag(&init_task, TIF_NOHZ);
> > +
> > + /*
> > + * There shouldn't be any thread at this early boot stage
> > + * but the scheduler is ready to host any. So lets walk
> > + * the tasklist just in case. tasklist_lock isn't necessary
> > + * either that early but take it for correctness checkers.
> > + */
> > + read_lock(&tasklist_lock);
> > + for_each_process_thread(p, t)
> > + set_tsk_thread_flag(t, TIF_NOHZ);
>
> If there should not be any task, should there not be a WARN_ON_ONCE()
> here?

Well, it's legal to have a task at that time because sched_init() was called.
I just haven't observed any task other than init/0. But future code (or alternate
configs than mine) might create a task between sched_init() and tick_init(). And
the above code takes care of such a possibility.

>
> > + read_unlock(&tasklist_lock);
> > +
> > + initialized = true;
> > }


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