Date: Fri, 11 Jul 2014 22:45:05 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Christoph Lameter <cl@gentwo.org>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
        peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, oleg@redhat.com,
        sbw@mit.edu
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 01:35:13PM -0700, Paul E. McKenney wrote:
> On Fri, Jul 11, 2014 at 09:11:15PM +0200, Frederic Weisbecker wrote:
> > On Fri, Jul 11, 2014 at 02:05:08PM -0500, Christoph Lameter wrote:
> > > On Fri, 11 Jul 2014, Frederic Weisbecker wrote:
> > > 
> > > > > 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.
> > > 
> > > Yeah.
> > > 
> > > > > 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.
> > > 
> > > The use case is pretty straightforward because we are trying to keep as
> > > much OS noise as possible off most processors. Processor 0 is the
> > > sacrificial lamb that will be used for all OS processing and hopefully all
> > > high latency operations will occur there. Processors 1-X have a tick but
> > > we still try to keep latencies sane. And then there is X-Y where tick is
> > > off.
> > 
> > Ok. I don't entirely get why you need 1-X but I can easily imagine some non-latency-critical
> > stuff running there.
> > 
> > Paul proposed "housekeeping=". If we ever go there, I'd rather vote for "sacrifical_lamb="
> 
> Given Christoph's desire for only one housekeeping CPU, I guess the
> counting version makes the most sense, so that "housekeeping=N" designates
> the first N non-nohz CPUs in numerical order as housekeeping CPUs.
> If there are fewer than N non-nohz CPUs, you get a splat at boot time
> and your request is capped at the number of non-nohz CPUs.
> 
> Seem reasonable?

I wonder if it's wouldn't be rather reasonable to affine housekeeping to all non-nohz-full CPUs
by default and then people who want finergrained housekeeping can affine manually kthreads from userspace.

That implies to bind without PF_NO_SETAFFINIT but that's easy enough to do.

> 
> 							Thanx, Paul
> 
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