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