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

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Message-ID: <alpine.DEB.2.11.1407111403090.5070@gentwo.org>
Date:	Fri, 11 Jul 2014 14:05:08 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, 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.
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