On Mon, 2007-10-01 at 19:43 +1000, Rusty Russell wrote: > On Fri, 2007-09-28 at 16:00 -0700, Dave Hansen wrote: > > Module refcounts currently use a percpu counter stored > > in the 'struct module'. However, we also have a more > > generic implementation that does stuff like handle > > hotplug cpus. > > > > I'm not actually all that convinced that this refcount > > actually does a lot of good, with cpus racing bumping > > the counters at the same time that they're being > > summed up. But, it certainly isn't any worse than > > what was there before. > > That's why we look at the counters inside stop_machine_run().
Ahhh. That makes sense. Although it wasn't apparent during my 3-second perusal of the code. > Note that (1) the module implementation handles hotplug CPUs You're saying it handles hotplug because of stop_machine_run()? > But it might be a useful cleanup (although a slight de-optimization). > If you want I'll queue for 2.6.24 (there are several other module > patches pending too). Might as well. It removed a very small amount of code, and opens the door a bit for future optimizations in a single place. > In an ideal world, (1) we would have percpu pointers using the same > percpu mechanism as percpu variables, (2) we would have a modal variant > of percpu counters which would collapse to a single counter when we > cared about the precise value (probably using stop_machine for the > transition). This would be useful for many other cases. Yeah, but before we do that, we need some kind of flag to get the percpu_counter_mod() fast path shoved into the slow path that takes the lock. I'm not sure the stop_machine() mechanism will work very well if we try to expand this much further for other users. What would the SGI guys think if these happened more than once in a blue moon? -- Dave - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
