On Tue, Jan 08, 2013 at 03:26:11PM -0500, Steven Rostedt wrote:
> On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:
>
> > diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> > index c952770..bd461ad 100644
> > --- a/kernel/context_tracking.c
> > +++ b/kernel/context_tracking.c
> > @@ -56,7 +56,7 @@ void user_enter(void)
> > local_irq_save(flags);
> > if (__this_cpu_read(context_tracking.active) &&
> > __this_cpu_read(context_tracking.state) != IN_USER) {
> > - __this_cpu_write(context_tracking.state, IN_USER);
> > + vtime_user_enter(current);
> > /*
> > * At this stage, only low level arch entry code remains and
> > * then we'll run in userspace. We can assume there won't be
> > @@ -65,6 +65,7 @@ void user_enter(void)
> > * on the tick.
> > */
> > rcu_user_enter();
>
> Hmm, the rcu_user_enter() can do quite a bit. Too bad we are accounting
> it as user time. I wonder if we could move the vtime_user_enter() below
> it. But then if vtime_user_enter() calls rcu_read_lock() it breaks.
If RCU_FAST_NO_HZ=y, the current mainline rcu_user_enter() can be a
bit expensive. It is going on a diet for 3.9, however.
But there is a lower limit because the CPU moving to adaptive-tick user
mode must reliably inform other CPUs of this, which involves some
overhead due to memory-ordering issues.
Thanx, Paul
> The notorious chicken vs egg ordeal!
>
> -- Steve
>
> > + __this_cpu_write(context_tracking.state, IN_USER);
> > }
> > local_irq_restore(flags);
> > }
> > @@ -90,12 +91,13 @@ void user_exit(void)
> >
> > local_irq_save(flags);
> > if (__this_cpu_read(context_tracking.state) == IN_USER) {
> > - __this_cpu_write(context_tracking.state, IN_KERNEL);
> > /*
> > * We are going to run code that may use RCU. Inform
> > * RCU core about that (ie: we may need the tick again).
> > */
> > rcu_user_exit();
> > + vtime_user_exit(current);
> > + __this_cpu_write(context_tracking.state, IN_KERNEL);
> > }
> > local_irq_restore(flags);
> > }
>
>
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