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Date: Mon, 25 Jan 2010 10:12:03 -0500 From: Steven Rostedt <rostedt@...dmis.org> To: paulmck@...ux.vnet.ibm.com 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, Valdis.Kletnieks@...edu, dhowells@...hat.com Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU On Sun, 2010-01-24 at 19:48 -0800, Paul E. McKenney wrote: > +/* > + * Check to see if any future RCU-related work will need to be done > + * by the current CPU, even if none need be done immediately, returning > + * 1 if so. This function is part of the RCU implementation; it is -not- > + * an exported member of the RCU API. > + * > + * Because we are not supporting preemptible RCU, attempt to accelerate > + * any current grace periods so that RCU no longer needs this CPU, but > + * only if all other CPUs are already in dynticks-idle mode. This will > + * allow the CPU cores to be powered down immediately, as opposed to after > + * waiting many milliseconds for grace periods to elapse. > + */ > +int rcu_needs_cpu(int cpu) > +{ > + int c = 1; > + int i; > + int thatcpu; > + > + /* Don't bother unless we are the last non-dyntick-idle CPU. */ > + for_each_cpu(thatcpu, nohz_cpu_mask) > + if (thatcpu != cpu) > + return rcu_needs_cpu_quick_check(cpu); > + > + /* Try to push remaining RCU-sched and RCU-bh callbacks through. */ > + for (i = 0; i < RCU_NEEDS_CPU_FLUSHES && c; i++) { > + c = 0; > + if (per_cpu(rcu_sched_data, cpu).nxtlist) { > + c = 1; > + rcu_sched_qs(cpu); > + force_quiescent_state(&rcu_sched_state, 0); > + __rcu_process_callbacks(&rcu_sched_state, > + &per_cpu(rcu_sched_data, cpu)); > + } > + if (per_cpu(rcu_bh_data, cpu).nxtlist) { > + c = 1; > + rcu_bh_qs(cpu); > + force_quiescent_state(&rcu_bh_state, 0); > + __rcu_process_callbacks(&rcu_bh_state, > + &per_cpu(rcu_bh_data, cpu)); > + } > + } > + > + /* If RCU callbacks are still pending, RCU still needs this CPU. */ > + return c; What happens if the last loop pushes out all callbacks? Then we would be returning 1 when we could really be returning 0. Wouldn't a better answer be: return per_cpu(rcu_sched_data, cpu).nxtlist || per_cpu(rcu_bh_data, cpu).nxtlist; -- Steve > +} > + > +#endif -- 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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