Messages in this thread | | | Date | Fri, 11 Jul 2014 12:08:16 -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 08:57:33PM +0200, Frederic Weisbecker wrote: > 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.
Agreed, this is only a problem when people run workloads for which NO_HZ_FULL is not well-suited. Which is why I settled on designating the non-nohz_full= CPUs as the housekeeping CPUs -- people wanting to run general workloads not suited to NO_HZ_FULL probably won't specify nohz_full=. If they don't, then any CPU can be a housekeeping CPU.
> > > > 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.
Hmmm... That probably means that we need boot-time parameters to make sysidle detection really happen. Otherwise, many users will get a nasty surprise once CONFIG_NO_HZ_FULL_SYSIDLE=y is enabled on systems that really aren't running HPC or RT workloads.
I suppose that I could confine SYSIDLE's attention to the nohz_full= CPUs -- that might actually make things work nicely in all cases with no configuration of any sort required. I will need to give this some thought.
> > 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.
Indeed! ;-)
Thanx, Paul
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