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Date: Mon, 16 Nov 2015 18:53:42 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Josh Triplett <josh@...htriplett.org> Cc: Jacob Pan <jacob.jun.pan@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, John Stultz <john.stultz@...aro.org>, LKML <linux-kernel@...r.kernel.org>, Arjan van de Ven <arjan@...ux.intel.com>, Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>, Len Brown <len.brown@...el.com>, Rafael Wysocki <rafael.j.wysocki@...el.com>, Eduardo Valentin <edubezval@...il.com>, Paul Turner <pjt@...gle.com> Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry On Mon, Nov 16, 2015 at 05:41:03PM -0800, Josh Triplett wrote: > On Mon, Nov 16, 2015 at 03:26:40PM -0800, Paul E. McKenney wrote: > > On Mon, Nov 16, 2015 at 02:32:11PM -0800, Josh Triplett wrote: > > > On Mon, Nov 16, 2015 at 01:51:26PM -0800, Jacob Pan wrote: > > > > On Mon, 16 Nov 2015 16:06:57 +0100 (CET) > > > > Thomas Gleixner <tglx@...utronix.de> wrote: > > > > > > > > > > <idle>-0 [000] 30.093474: bprint: > > > > > > __tick_nohz_idle_enter: JPAN: tick_nohz_stop_sched_tick 609 delta > > > > > > 1000000 [JP] but sees delta is exactly 1 tick away. didn't stop > > > > > > tick. > > > > > > > > > > If the delta is 1 tick then it is not supposed to stop it. Did you > > > > > ever try to figure out WHY it is 1 tick? > > > > > > > > > > There are two code pathes which can set it to basemono + TICK_NSEC: > > > > > > > > > > if (rcu_needs_cpu(basemono, &next_rcu) || > > > > > arch_needs_cpu() || irq_work_needs_cpu()) { > > > > > next_tick = basemono + TICK_NSEC; > > > > > } else { > > > > > next_tmr = get_next_timer_interrupt(basejiff, > > > > > basemono); ts->next_timer = next_tmr; > > > > > /* Take the next rcu event into account */ > > > > > next_tick = next_rcu < next_tmr ? next_rcu : next_tmr; > > > > > } > > > > > > > > > > Can you please figure out WHY the tick is requested to continue > > > > > instead of blindly wreckaging the logic in that code? > > > > > > > > Looks like the it hits in both cases during forced idle. > > > > + Josh > > > > + Paul > > > > > > > > For the first case, it is always related to RCU. I found there are two > > > > CONFIG options to avoid this undesired tick in idle loop. > > > > 1. enable CONFIG_RCU_NOCB_CPU_ALL, offload to orcu kthreads > > > > 2. or enable CONFIG_RCU_FAST_NO_HZ (enter dytick idle w/ rcu callback) > > > > > > > > Either one works but my concern is that users may not realize the > > > > intricate CONFIG_ options and how they translate into energy savings. > > > > Consulted with Josh, it seems we could add a check here to recognize > > > > the forced idle state and relax rcu_needs_cpu() to return false even it > > > > has callbacks. Since we are blocking everybody for a short time (5 ticks > > > > default). It should not impact synchronize and kfree rcu. > > > > > > Right; as long as you're blocking *everybody*, and RCU priority boosting > > > doesn't come into play (meaning a real-time task is waiting on RCU > > > callbacks), then I don't see any harm in blocking RCU callbacks for a > > > while. You'd block completion of synchronize_rcu() and similar, as well > > > as memory reclamation, but since you've blocked *every* CPU systemwide > > > then that doesn't cause a problem. > > > > True enough. But how does RCU distinguish between this being a > > normal idle cycle that might last indefinitely on the one hand and the > > five-jiffy system-wide throttling on the other? OK, maybe there is a > > global variable that says that the just-now-starting idle period is > > system-wide throttling. But then what about the CPU that just went > > idle 10 microseconds ago, and therefore left its timer tick running? > > Fine and well, we could IPI it to wake it up and let it see that we > > are now doing thermal throttling. But then we presumably also have to > > IPI it at the end of the thermal-throttling interval in order for it to > > re-evaluate whether or not it should have the tick going. :-/ > > > > On the one hand, I am sure that all of this can be made to work, > > but simply having systems using thermal throttling enable either > > CONFIG_RCU_NOCB_CPU_ALL or CONFIG_RCU_FAST_NO_HZ seems -way- simpler. > > CONFIG_RCU_FAST_NO_HZ is probably the better choice for generic workloads, > > but CONFIG_RCU_NOCB_CPU_ALL is the better choice for embedded workloads > > where it is less likely that RCU callbacks will be posted with continuous > > wild abandon. > > > > Or am I missing something subtle here? > > I agree that it seems preferable to make this require an existing RCU > solution rather than adding more complexity to the RCU idle path. One > possible thing that may affect the choice of solution: this needs to > idle *every* CPU, without leaving any CPU awake to handle callbacks or > similar. Fair point. When in the five-jiffy throttling state, what can wake up a CPU? In an earlier version of this proposal, the answer was "nothing", but maybe that has changed. 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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