Messages in this thread | | | Date | Mon, 1 Jul 2013 12:16:57 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC nohz_full v2 2/7] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state |
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On Mon, Jul 01, 2013 at 11:34:13AM -0700, Josh Triplett wrote: > On Mon, Jul 01, 2013 at 11:23:26AM -0700, Paul E. McKenney wrote: > > On Mon, Jul 01, 2013 at 11:16:01AM -0700, Josh Triplett wrote: > > > On Mon, Jul 01, 2013 at 08:52:20AM -0700, Paul E. McKenney wrote: > > > > On Mon, Jul 01, 2013 at 08:31:50AM -0700, Josh Triplett wrote: > > > > > On Fri, Jun 28, 2013 at 01:10:17PM -0700, Paul E. McKenney wrote: > > > > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > > > > > > > > > > > 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. > > > > > > > > > > Can you explain, both in the commit messages and in the comments added > > > > > by the next commit, *why* this code doesn't consider userspace a > > > > > quiescent state? > > > > > > > > Good point! Does the following explain it? > > > > > > > > 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. > > > > > > Good explanation, thanks. > > > > > > This also naturally raises the question "How can we let userspace get > > > accurate time without forcing a timer tick?". > > > > We don't. ;-) > > We don't currently, hence my question about how we can. :)
Per-CPU atomic clocks? Hardware-synchronized time across all CPUs? Hardware detection of the full-system idle state, allowing the hardware synchronization to be shut down in that case? (But of course started with full synchronization whenever something went non-idle!) Use a periodic hrtimer instead of the scheduling-clock tick? (Aside from the fact that the scheduling-clock tick is already an hrtimer in some configurations...)
The last might not be as silly as it sounds. I believe that timekeeping can tolerate an interrupt rate much slower than HZ, so if the timekeeping CPU figured out that the only reason for the scheduling-clock tick was timekeeping, it could run the tick much more slowly. That said, I wouldn't blame Frederic for deferring that particular increment of complexity for a bit. ;-)
> > Without CONFIG_NO_HZ_FULL, if a CPU is running in user mode, that CPU > > takes scheduling-clock interrupts. User-mode code will therefore always > > see accurate time. For some definition of "accurate", anyway. > > > > With CONFIG_NO_HZ_FULL and without CONFIG_NO_HZ_FULL_SYSIDLE, a single > > designated CPU will always be taking scheduling-clock interrupts, which > > again ensures that user-mode code will always see accurate time. > > > > With both CONFIG_NO_HZ_FULL and CONFIG_NO_HZ_FULL_SYSIDLE, if > > any CPU other than the timekeeping CPU is nonidle (where "nonidle" > > includes usermode execution), then the timekeeping CPU will be taking > > scheduling-clock interrupts, yet again ensuring that user-mode code will > > always see accurate time. If all CPUs are idle (in other words, we are > > in RCU_SYSIDLE_FULL_NOTED state and the timekeeping CPU is also idle), > > scheduling-clock interrupts will be globally disabled. Or will be, > > once I fix the bug noted by Frederic. > > > > I am guessing that you would like this added to the explanation? ;-) > > Seemed pretty clear already from your previous explanation above, but > since you've taken the time to write it... :)
If the above sufficed, the additional verbiage might add more confusion than understanding. ;-)
Thanx, Paul
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