On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <[email protected]> > > 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 <[email protected]> > Acked-by: Frederic Weisbecker <[email protected]> > Cc: Steven Rostedt <[email protected]>
One comment below. With that change: Reviewed-by: Josh Triplett <[email protected]> > +#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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
