linux-kernel - Re: [PATCH 4/6] nohz: support PR_DATAPLANE

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Message-ID: <20150513175150.GL6776@linux.vnet.ibm.com>
Date:	Wed, 13 May 2015 10:51:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] nohz: support PR_DATAPLANE_QUIESCE

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@...nel.org> wrote:
> >
> > * Peter Zijlstra <peterz@...radead.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

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