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Date: Wed, 27 Jan 2010 02:01:36 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Andi Kleen <andi@...stfloor.org> Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU On Wed, Jan 27, 2010 at 10:43:36AM +0100, Andi Kleen wrote: > On Tue, Jan 26, 2010 at 09:20:50PM -0800, Paul E. McKenney wrote: > > On Tue, Jan 26, 2010 at 10:30:57PM +0100, Andi Kleen wrote: > > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes: > > > > > > Kind of offtopic to the original patch, but I couldn't > > > resist... > > > > > > > +config RCU_FAST_NO_HZ > > > > + bool "Accelerate last non-dyntick-idle CPU's grace periods" > > > > + depends on TREE_RCU && NO_HZ && SMP > > > > > > Having such a thing as a config option doesn't really make > > > any sense to me. Who would want to recompile their kernel > > > to enable/disable this? If anything it should be runtime, or better > > > just unconditionally on. > > > > It adds significant overhead on entry to dyntick-idle mode for systems > > with large numbers of CPUs. :-( > > Can't you simply check that at runtime then? > > if (num_possible_cpus() > 20) > ... > > BTW the new small is large. This years high end desktop PC will come with > upto 12 CPU threads. It would likely be challenging to find a good > number for 20 that holds up with the future. And this was another line of reasoning that lead me to the extra kernel config parameter. > Or better perhaps have some threshold that you don't do it > that often, or only do it when you expect to be idle for a long > enough time that the CPU can enter deeper idle states > > (I higher idle states some more wakeups typically don't matter > that much) > > The cpufreq/cstate governour have a reasonable good idea > now how "idle" the system is and will be. Maybe you can reuse > that information somehow. My first thought was to find an existing "I am a small device running on battery power" or "low power consumption is critical to me" config parameter. I didn't find anything that looked like that. If there was one, I would make RCU_FAST_NO_HZ depend on it. Or did I miss some kernel parameter or API? 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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