Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime accounting

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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);
> >  }
> 
>