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" <paul...@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?". - Josh Triplett -- 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/
