Messages in this thread | | | Subject | Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU | From | Steven Rostedt <> | Date | Mon, 25 Jan 2010 10:12:03 -0500 |
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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
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