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Message-ID: <20140712013941.GV16041@linux.vnet.ibm.com> Date: Fri, 11 Jul 2014 18:39:41 -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 10:45:05PM +0200, Frederic Weisbecker wrote: > 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. Works for me! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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