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Message-ID: <20140721170459.GP3935@laptop> Date: Mon, 21 Jul 2014 19:04:59 +0200 From: Peter Zijlstra <peterz@...radead.org> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Frederic Weisbecker <fweisbec@...il.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, tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, oleg@...hat.com, bobby.prani@...il.com Subject: Re: [PATCH tip/core/rcu] Do not keep timekeeping CPU tick running for non-nohz_full= CPUs On Mon, Jul 21, 2014 at 08:57:41AM -0700, Paul E. McKenney wrote: > On Sun, Jul 20, 2014 at 10:34:17PM +0200, Peter Zijlstra wrote: > > On Sun, Jul 20, 2014 at 04:47:59AM -0700, Paul E. McKenney wrote: > > > So we really have to have -all- the CPUs be idle to turn off the timekeeper. > > > > That seems to be pretty unavoidable any which way around. > > Hmmm... The exception would be the likely common case where none of > the CPUs are flagged as nohz_full= CPUs. If we handled that case as > if CONFIG_NO_HZ_FULL=n, we would have handled almost all of > the problem. You mean that is not currently the case? Yes that seems like a fairly sane thing to do. > > > This won't make the battery-powered embedded guys happy... > > > > > > Other thoughts on this? We really should not be setting > > > CONFIG_NO_HZ_FULL_SYSIDLE by default until this is solved. > > > > What are those same guys doing with nohz_full to begin with? > > If CONFIG_NO_HZ_FULL_SYSIDLE=y is the default, my main concern is for > people who didn't really want it, and who thus did not set the nohz_full= > boot parameter. Hence my suggestion above that we treat that case as > if CONFIG_NO_HZ_FULL=n (and thus also as if CONFIG_NO_HZ_FULL_SYSIDLE=n). ack > There have been some people saying that they want only a subset of > their CPUs in nohz_full= state, and these guys seem to want to run a > mixed workload. For example, they have HPC (or RT) workloads on the > nohz_full= CPUs, and also want normal high-throughput processing on the > remaining CPUs. If software was trivial (and making other unlikely > assumptions about the perfection of the world and the invalidity of > Murphy's lawy), we would want the timekeeping CPU to be able to move > among the non-nohz_full= CPUs. Yeah, I don't see a problem with that, but then I'm not entirely sure why we use RCU to track system idle state. > However, this should be a small fraction of the users, and many of > these guys would probably be open to making a few changes. Thus, a > less-proactive approach should allow us to solve their actual problems, as > opposed to the problems that we speculate that they might encounter. ;-) But you still haven't talked about the battery people... I don't think nohz_full is something they should care about / use. -- 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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