Messages in this thread |  | | Date | Tue, 8 Jan 2013 13:00:35 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 03/33] cputime: Generic on-demand virtual cputime accounting |
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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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