On Sun, Aug 18, 2013 at 06:39:25PM -0700, Paul E. McKenney wrote: > On Sat, Aug 17, 2013 at 08:09:21PM -0700, Josh Triplett wrote: > > On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote: > > > From: "Paul E. McKenney" <[email protected]> > > > > > > This commit adds control variables and states for full-system idle. > > > The system will progress through the states in numerical order when > > > the system is fully idle (other than the timekeeping CPU), and reset > > > down to the initial state if any non-timekeeping CPU goes non-idle. > > > The current state is kept in full_sysidle_state. > > > > > > A RCU_SYSIDLE_SMALL macro is defined, and systems with this number > > > of CPUs or fewer move through the states more aggressively. The idea > > > is that the resulting memory contention is less of a problem on small > > > systems. Architectures can adjust this value (which defaults to 8) > > > using CONFIG_ARCH_RCU_SYSIDLE_SMALL. > > > > > > One flavor of RCU will be in charge of driving the state machine, > > > defined by rcu_sysidle_state. This should be the busiest flavor of RCU. > > > > > > Signed-off-by: Paul E. McKenney <[email protected]> > > > Cc: Frederic Weisbecker <[email protected]> > > > Cc: Steven Rostedt <[email protected]> > > > > One issue (and one question) below; with the issue addressed, > > Reviewed-by: Josh Triplett <[email protected]> > > > > > kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++ > > > 1 file changed, 28 insertions(+) > > > > > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > > > index eab81da..64a05b9f 100644 > > > --- a/kernel/rcutree_plugin.h > > > +++ b/kernel/rcutree_plugin.h > > > @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu) > > > #ifdef CONFIG_NO_HZ_FULL_SYSIDLE > > > > > > /* > > > + * Handle small systems specially, accelerating their transition into > > > + * full idle state. Allow arches to override this code's idea of > > > + * what constitutes a "small" system. > > > + */ > > > +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL > > > > I don't see any Kconfig creating this new config option. > > > > Also, why not simply define this config option unconditionally, with a > > default of 8, and then use its value directly? > > Good point, removing this and adding a Kconfig option in the > "nohz_full: Add full-system-idle state machine" commit, with a > default value of 8. Architecture maintainers who want something > different can then set that up in their defconfig files.
Sounds good. > > > +static int __maybe_unused full_sysidle_state; /* Current system-idle > > > state. */ > > > +#define RCU_SYSIDLE_NOT 0 /* Some CPU is not idle. */ > > > +#define RCU_SYSIDLE_SHORT 1 /* All CPUs idle for brief > > > period. */ > > > +#define RCU_SYSIDLE_LONG 2 /* All CPUs idle for long enough. */ > > > +#define RCU_SYSIDLE_FULL 3 /* All CPUs idle, ready for sysidle. */ > > > +#define RCU_SYSIDLE_FULL_NOTED 4 /* Actually entered sysidle > > > state. */ > > > > Perhaps there's a kernel style rule I'm not thinking of that makes it > > verboten, but: why not use an enum for a state variable like this? > > I didn't trust enum interactions with xchg and cmpxchg, so opted for "int" > instead. That said, enum is much more portable than when I last looked > at it. Admittedly, the last time I looked at it was in the early 1980s... That would make sense if this was an atomic_t, but it's an int; unless I missed something, you don't currently use xchg or cmpxchg on it. - 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/
