Messages in this thread | | | Date | Wed, 23 Feb 2011 17:16:48 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread |
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On Tue, Feb 22, 2011 at 05:39:40PM -0800, Paul E. McKenney wrote: > +/* > + * Wake up the current CPU's kthread. This replaces raise_softirq() > + * in earlier versions of RCU. > + */ > +static void invoke_rcu_kthread(void) > +{ > + unsigned long flags; > + wait_queue_head_t *q; > + int cpu; > + > + local_irq_save(flags); > + cpu = smp_processor_id(); > + if (per_cpu(rcu_cpu_kthread_task, cpu) == NULL) { > + local_irq_restore(flags); > + return; > + } > + per_cpu(rcu_cpu_has_work, cpu) = 1; > + q = &per_cpu(rcu_cpu_wq, cpu);
I see you make extensive use of per_cpu() accessors even for local variables.
I tend to think it's better to use __get_cpu_var() for local accesses and keep per_cpu() for remote accesses.
There are several reasons for that:
* __get_cpu_var() checks we are in a non-preemptible section, per_cpu() doesn't. That may sound of a limited interest for code like the above, but by the time code can move, and then we might lose track of some things, etc...
* local accesses can be optimized by architectures. per_cpu() implies finding the local cpu number, and dereference an array of cpu offsets with that number to find the local cpu offset. __get_cpu_var() does a direct access to __my_cpu_offset which is a nice shortcut if the arch implements it.
* It makes code easier to review: we know that __get_cpu_var() is for local accesses and per_cpu() for remote.
> + wake_up(q); > + local_irq_restore(flags); > +}
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