Messages in this thread | | | Date | Fri, 11 Jul 2014 11:45:28 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs |
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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.
> > 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.
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.
Thanx, Paul
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