On Wed, Aug 12, 2015 at 09:03:42AM -0700, Paul E. McKenney wrote: > On Wed, Aug 12, 2015 at 04:27:34PM +0200, Frederic Weisbecker wrote: > > On Tue, Aug 11, 2015 at 08:42:58PM +0200, Luis R. Rodriguez 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. > > > > > > > > - 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. > > > > > > > > - vtime. Beats the heck out of me. > > > > > > > > - Whatever deferred things Christoph keeps reminding us about. > > > > > > > > Honestly, I don't fully understand what all these hooks are supposed > > > > to do, nor do I care all that much. From my perspective, the code > > > > code should be able to do whatever it wants and rely on appropriate > > > > notifications from arch code. It would be great if we could come up > > > > with something straightforward that covers everything. For example: > > > > > > > > user_mode_to_kernel_mode() > > > > kernel_mode_to_user_mode() > > > > kernel_mode_to_guest_mode() > > > > in_a_periodic_tick() > > > > starting_nmi() > > > > ending_nmi() > > > > may_i_turn_off_ticks_right_now() > > > > or, better yet: > > > > i_am_turning_off_ticks_right_now_and_register_your_own_darned_hrtimer_if_thats_a_problem() > > > > > > > > Some arches may need: > > > > > > > > i_am_lame_and_forgot_my_previous_context() > > > > > > Can all this information be generalized with some basic core hooks > > > or could some of this contextual informatioin typically vary depending > > > on the sequence we are in ? It sounds like its the later and that's > > > the issue ? > > > > That's what we do with context tracking. It tracks the context (user/kernel) > > and stores these informations. And indeed the contextual informations can > > vary > > depending for example if an exception triggered in userspace or kernelspace. > > Another question of interest is "Can things be arranged so that RCU uses > the context-tracking information directly in place of rcu_dynticks?" > In theory, the answer is clearly "yes", but the reason that RCU's > accounting is heavyweight is the need to get precise state readout on > other CPUs. So it is quite possible that making RCU directly use the > context-tracking information will make that tracking slower and more > complex, so that the overall effect will be zero net improvement.
Yeah, that's partly what I meant by "it's possible, but we might not be proud of the result". > But it does seem worth a look. Sure. -- 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/
