linux-kernel - Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ

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Message-ID: <20140711203513.GA10652@linux.vnet.ibm.com>
Date:	Fri, 11 Jul 2014 13:35:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...two.org>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to
 non-NO_HZ_FULL CPUs

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?

							Thanx, Paul

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