Messages in this thread | | | Subject | Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU | From | Steven Rostedt <> | Date | Wed, 27 Jan 2010 09:52:32 -0500 |
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On Wed, 2010-01-27 at 06:11 -0800, Paul E. McKenney wrote: > On Mon, Jan 25, 2010 at 10:12:03AM -0500, Steven Rostedt wrote: > > 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; > > Good point!!! > > Or I can move the assignment to "c" to the end of each branch of the > "if" statement, and do something like the following: > > c = !!per_cpu(rcu_sched_data, cpu).nxtlist;
Hmm, that may just add obfuscation to those looking at the code.
> > But either way, you are right, it does not make sense to go to all the > trouble of forcing a grace period and then failing to take advantage > of it.
Yeah, whatever implementation is fine, as long as it works and takes advantage of all forced grace periods.
-- Steve
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