On Tue, Aug 11, 2015 at 12:07:54PM -0700, Andy Lutomirski wrote: > On Tue, Aug 11, 2015 at 11:33 AM, Paul E. McKenney > <paul...@linux.vnet.ibm.com> wrote: > > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote: > >> This is a bit late, but here goes anyway. > >> > >> Having played with the x86 context tracking hooks for awhile, I think > >> it would be nice if core code that needs to be aware of CPU context > >> (kernel, user, idle, guest, etc) could come up with single, > >> comprehensible, easily validated set of hooks that arch code is > >> supposed to call. > >> > >> Currently we have: > >> > >> - RCU hooks, which come in a wide variety to notify about IRQs, NMIs, etc. > > > > Something about people yelling at me for waking up idle CPUs, thus > > degrading their battery lifetimes. ;-) > > > >> - Context tracking hooks. Only used by some arches. Calling these > >> calls the RCU hooks for you in most cases. They have weird > >> interactions with interrupts and they're slow. > > > > Combining these would be good, but there are subtleties. For example, > > some arches don't have context tracking, but RCU still needs to correctly > > identify idle CPUs without in any way interrupting or awakening that CPU. > > It would be good to make this faster, but it does have to work. > > Could we maybe have one set of old RCU-only (no context tracking) > callbacks and a completely separate set of callbacks for arches that > support full context tracking? The implementation of the latter would > presumably call into RCU.
That's already what we do I think. rcu_idle_enter()/rcu_idle_exit() are the old RCU-only stuffs and the rest (rcu_user_exit()/enter()) uses context tracking. > > >> may_i_turn_off_ticks_right_now() > > > > This is RCU if CONFIG_RCU_FAST_NO_HZ=n. > > > >> or, better yet: > >> i_am_turning_off_ticks_right_now_and_register_your_own_darned_hrtimer_if_thats_a_problem() > > > > This is RCU if CONFIG_RCU_FAST_NO_HZ=y. It would not be difficult to > > make RCU do this if CONFIG_RCU_FAST_NO_HZ=n as well, but doing so would > > increase to/from idle overhead. > > If things actually end up using hrtimers, we might also want > get_off_my_lawn() aka "isolate this cpu now and try to do all the > deferred stuff right now and kill off those hrtimers". Yeah that's what we are trying to do. But hrtimers aren't special here, they are noise just like any other. > > Rik is (was?) trying to make some housekeeper CPU probe other CPUs' > state to eliminate the need for exact vtime accounting and thus speed > up transitions to/from user or idle. Only user. And that's only about vtime. RCU still needs to be handled locally. > It would be really neat if we > could simultaneously have quick idle/user transitions *and* avoid > deferred per-cpu work interrupting idle/user mode. I think that's the goal. If we eventually offline the vtime accounting, all that remains is RCU hooks on user/kernel transitions. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
