Messages in this thread | | | Date | Wed, 23 Feb 2011 18:30:06 +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 Wed, Feb 23, 2011 at 12:03:56PM -0500, Mathieu Desnoyers wrote: > * Steven Rostedt (rostedt@goodmis.org) wrote: > > On Wed, 2011-02-23 at 17:16 +0100, Frederic Weisbecker wrote: > > > 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... > > > > Ah, but so does smp_processor_id() ;-) > > > > > > > > * 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. > > [Adding Christoph Lameter to CC list] > > This is not quite true on x86_64 and s390 anymore. __get_cpu_var() now > uses a segment selector override to get the local CPU variable on x86. > See x86's percpu.h for details. > > So even performance-wise, using __get_cpu_var() over per_cpu() should be > a win on widely used architectures nowadays,
Looking at x86_64, it indeed optimizes further by overriding this_cpu_ptr(). It does the same than the generic this_cpu_ptr() on an overriden my_cpu_offset, but it also economizes a temporary store.
> > > > > True, but we could also argue that the multiple checks for being preempt > > can also be an issue. > > At least on x86 preemption don't actually need to be disabled: selection > of the right per-cpu memory location is done atomically with the rest of > the instruction by the segment selector.
It depends on the case, you may still need to disable preemption if you use your variable further than just a quick op, which is often the case.
That's up to this_cpu_add() op things, depending on what the arch is capable of wrt. local atomicity.
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