Messages in this thread | | | Date | Wed, 23 Feb 2011 12:03:56 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq to kthread |
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* 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, thanks to Christoph's work on this_cpu accessors.
> > 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 makes code easier to review: we know that __get_cpu_var() is > > for local accesses and per_cpu() for remote. > > This I'll agree with you. > > In the past, I've thought about which one is better (per_cpu() vs > __get_cpu_var()). > > But, that last point is a good one. Just knowing that this is for the > local CPU helps with the amount of info your mind needs to process when > looking at this code. And the mind needs all the help it can get when > reviewing this code ;-) >
Agreed, better documentation of the code is also a win.
Thanks,
Mathieu
> -- Steve > > > > > > + wake_up(q); > > > + local_irq_restore(flags); > > > +} > >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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