Re: [PATCH 4/6] nohz: support PR_DATAPLANE_QUIESCE

Paul E. McKenney
2015-05-13T21:00:27Z

On Tue, May 12, 2015 at 09:35:25PM -0700, Andy Lutomirski wrote:
> On Tue, May 12, 2015 at 5:52 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Peter Zijlstra <peterz@infradead.org> wrote:
> >
> >> > So if then a prctl() (or other system call) could be a shortcut
> >> > to:
> >> >
> >> > - move the task to an isolated CPU
> >> > - make sure there _is_ such an isolated domain available
> >> >
> >> > I.e. have some programmatic, kernel provided way for an
> >> > application to be sure it's running in the right environment.
> >> > Relying on random administration flags here and there won't cut
> >> > it.
> >>
> >> No, we already have sched_setaffinity() and we should not duplicate
> >> its ability to move tasks about.
> >
> > But sched_setaffinity() does not guarantee isolation - it's just a
> > syscall to move a task to a set of CPUs, which might be isolated or
> > not.
> >
> > What I suggested is that it might make sense to offer a system call,
> > for example a sched_setparam() variant, that makes such guarantees.
> >
> > Say if user-space does:
> >
> > ret = sched_setscheduler(0, BIND_ISOLATED, &isolation_params);
> >
> > ... then we would get the task moved to an isolated domain and get a 0
> > return code if the kernel is able to do all that and if the current
> > uid/namespace/etc. has the required permissions and such.
> >
> > ( BIND_ISOLATED will not replace the current p->policy value, so it's
> > still possible to use the regular policies as well on top of this. )
>
> I think we shouldn't have magic selection of an isolated domain.
> Anyone using this has already configured some isolated CPUs and
> probably wants to choose the CPU and, especially, NUMA node
> themselves. Also, maybe it should be a special type of realtime
> class/priority -- doing this should require RT permission IMO.

I have no real argument against special permissions, but this feature
is totally orthogonal to realtime classes/priorities. It is perfectly
legitimate for a given CPU's single runnable task to be SCHED_OTHER,
for example.

Thanx, Paul