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

On Tue, May 12, 2015 at 5:52 AM, Ingo Molnar [off-list ref] wrote:

* Peter Zijlstra [off-list ref] wrote:

quoted

quoted

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.

--Andy