On Fri, Jul 11, 2014 at 11:45:28AM -0700, Paul E. McKenney wrote: > On Fri, Jul 11, 2014 at 08:25:43PM +0200, Frederic Weisbecker wrote: > > On Fri, Jul 11, 2014 at 01:10:41PM -0500, Christoph Lameter wrote: > > > On Tue, 8 Jul 2014, Frederic Weisbecker wrote: > > > > > > > > I was figuring that a fair number of the kthreads might eventually > > > > > be using this, not just for the grace-period kthreads. > > > > > > > > Ok makes sense. But can we just rename the cpumask to housekeeping_mask? > > > > > > That would imply that all no-nohz processors are housekeeping? So all > > > processors with a tick are housekeeping? > > > > Well, now that I think about it again, I would really like to keep > > housekeeping > > to CPU 0 when nohz_full= is passed. > > When CONFIG_NO_HZ_FULL_SYSIDLE=y, then housekeeping kthreads are bound to > CPU 0. However, doing this causes significant slowdowns according to > Fengguang's testing, so when CONFIG_NO_HZ_FULL_SYSIDLE=n, I bind the > housekeeping kthreads to the set of non-nohz_full CPUs.
But did he see these slowdowns with nohz_full= parameter passed? I doubt he tested that. And I'm not sure that people who need full dynticks will run the usecases that trigger slowdowns with grace period kthreads. I also doubt that people will often omit other CPUs than CPU 0 nohz_full= range. > > > > Could we make that set configurable? Ideally I'd like to have the ability > > > restrict the housekeeping to one processor. > > > > Ah, I'm curious about your usecase. But I think we can do that. And we > > should. > > > > In fact I think that Paul could keep affining grace period kthread to CPU 0 > > for the sole case when we have nohz_full= parameter passed. > > > > I think the performance issues reported to him refer to CONFIG_NO_HZ_FULL=y > > config without nohz_full= parameter passed. That's the most important to > > address. > > > > Optimizing the "nohz_full= passed" case is probably not very useful and > > worse > > it complicate things a lot. > > > > What do you think Paul? Can we simplify things that way? I'm pretty sure > > that > > nobody cares about optimizing the nohz_full= case. That would really > > simplify > > things to stick to CPU 0. > > When we have CONFIG_NO_HZ_FULL_SYSIDLE=y, agreed. In that case, having > housekeeping CPUs on CPUs other than CPU 0 means that you never reach > full-system-idle state. That said I expect CONFIG_NO_HZ_FULL_SYSIDLE=y to be always enable for those who run NO_HZ_FULL in the long run. > > But in other cases, we appear to need more than one housekeeping CPU. > This is especially the case when people run general workloads on systems > that have NO_HZ_FULL=y, which appears to be a significant fraction of > the systems these days. Yeah NO_HZ_FULL=y is likely to be enabled in many distros. But you know the amount of nohz_full= users. -- 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/
