Messages in this thread | | | Date | Sat, 17 Aug 2013 20:02:34 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state |
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On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > This commit adds fields to the rcu_dyntick structure that are used to > detect idle CPUs. These new fields differ from the existing ones in > that the existing ones consider a CPU executing in user mode to be idle, > where the new ones consider CPUs executing in user mode to be busy. > The handling of these new fields is otherwise quite similar to that for > the exiting fields. This commit also adds the initialization required > for these fields. > > So, why is usermode execution treated differently, with RCU considering > it a quiescent state equivalent to idle, while in contrast the new > full-system idle state detection considers usermode execution to be > non-idle? > > It turns out that although one of RCU's quiescent states is usermode > execution, it is not a full-system idle state. This is because the > purpose of the full-system idle state is not RCU, but rather determining > when accurate timekeeping can safely be disabled. Whenever accurate > timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one > CPU must keep the scheduling-clock tick going. If even one CPU is > executing in user mode, accurate timekeeping is requires, particularly for > architectures where gettimeofday() and friends do not enter the kernel. > Only when all CPUs are really and truly idle can accurate timekeeping be > disabled, allowing all CPUs to turn off the scheduling clock interrupt, > thus greatly improving energy efficiency. > > This naturally raises the question "Why is this code in RCU rather than in > timekeeping?", and the answer is that RCU has the data and infrastructure > to efficiently make this determination. > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Acked-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: Steven Rostedt <rostedt@goodmis.org>
One comment below. With that change: Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE > + > +/* > + * Initialize dynticks sysidle state for CPUs coming online. > + */ > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) > +{ > + rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE; > +} > + > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */ > + > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) > +{ > +} > + > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
Just move the ifdef around the function body:
static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp) { #ifdef CONFIG_NO_HZ_FULL_SYSIDLE rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE; #endif /* CONFIG_NO_HZ_FULL_SYSIDLE */ }
- Josh Triplett
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