On Mon, Jul 08, 2013 at 06:30:01PM -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. > The handling of these new fields is otherwise quite similar to that for > the exiting fields. This commit also adds the initialization required > for these fields. > > So, why is usermode execution treated differently, with RCU considering > it a quiescent state equivalent to idle, while in contrast the new > full-system idle state detection considers usermode execution to be > non-idle? > > It turns out that 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.
but but but but... why doesn't the regular nohz code qualify? I'd think that too would be tracking pretty much the same things, no? -- 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/
