Messages in this thread | | | Date | Tue, 15 Nov 2011 19:30:29 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC tip/core/rcu 19/28] nohz: Allow rcu extended quiescent state handling seperately from tick stop |
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On Thu, Nov 10, 2011 at 09:22:19AM -0800, Paul E. McKenney wrote: > On Wed, Nov 09, 2011 at 05:48:11PM +0100, Frederic Weisbecker wrote: > > On Thu, Nov 03, 2011 at 09:06:56AM -0700, Paul E. McKenney wrote: > > > On Thu, Nov 03, 2011 at 08:31:02AM -0700, Josh Triplett wrote: > > > > On Thu, Nov 03, 2011 at 06:32:31AM -0700, Paul E. McKenney wrote: > > > > > On Thu, Nov 03, 2011 at 12:54:33PM +0100, Frederic Weisbecker wrote: > > > > > > On Wed, Nov 02, 2011 at 09:00:03PM -0700, Josh Triplett wrote: > > > > > > > On Wed, Nov 02, 2011 at 01:30:40PM -0700, Paul E. McKenney wrote: > > > > > > > > From: Frederic Weisbecker <fweisbec@gmail.com> > > > > > > > > > > > > > > > > It is assumed that rcu won't be used once we switch to tickless > > > > > > > > mode and until we restart the tick. However this is not always > > > > > > > > true, as in x86-64 where we dereference the idle notifiers after > > > > > > > > the tick is stopped. > > > > > > > > > > > > > > > > To prepare for fixing this, add two new APIs: > > > > > > > > tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu(). > > > > > > > > > > > > > > > > If no use of RCU is made in the idle loop between > > > > > > > > tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch > > > > > > > > must instead call the new *_norcu() version such that the arch doesn't > > > > > > > > need to call rcu_idle_enter() and rcu_idle_exit(). > > > > > > > > > > > > > > The _norcu names confused me a bit. At first, I thought they meant > > > > > > > "idle but not RCU idle, so you can use RCU", but from re-reading the > > > > > > > commit message, apparently they mean "idle and RCU idle, so don't use > > > > > > > RCU". What about something like _forbid_rcu instead? Or, > > > > > > > alternatively, why not just go ahead and separate the two types of idle > > > > > > > entirely rather than introducing the _norcu variants first? > > > > > > > > > > > > Or tick_nohz_idle_enter_rcu_stop() and tick_nohz_idle_exit_rcu_restart()? > > > > > > > > > > > > Sounds clear but too long. May be we can shorten the tick_nohz thing in the > > > > > > beginning. > > > > > > > > > > How about tick_nohz_rcu_idle_enter() vs. tick_nohz_idle_enter() on > > > > > entry to the idle loop and tick_nohz_rcu_idle_exit() vs > > > > > tick_nohz_idle_exit() on exit? > > > > > > > > > > That said, I don't feel all that strongly on this naming topic. > > > > > > > > Mostly I think that since this series tries to separate the concepts of > > > > "idle nohz" and "rcu extended quiescent state", we should end up with > > > > two entirely separate functions delimiting those two, without any > > > > functions that poke both with correspondingly complex compound names. > > > > > > Having four API members rather than the current six does seem quite > > > attractive to me. Frederic, any reason why this approach won't work? > > > > The approach I took might sound silly but it's mostly an optimization: > > > > I did the *_norcu() variant mostly to be able to keep rcu_idle_enter() > > call under the same local_irq_disable() section. > > > > This way we can't have an interrupt in between that can needlessly perform > > RCU work (and trigger the softirq in the worst case), delaying the point > > where we actually put the CPU to sleep. > > But we have to tolerate this sort of thing on some architectures (x86 > and Power) in order to allow idle-task use of RCU read-side primitives, > right? > > So consolidating from six to four APIs doesn't expand the overall state > space.
Well, we tolerate that, the two more APIs are there for optimization, not to provide correctness. But if you want me to remove the optimization and keep only the four APIs I can do it.
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