On Mon, 23 Sep 2013 16:54:46 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Mon, Sep 23, 2013 at 10:50:17AM -0400, Steven Rostedt wrote: > > On Thu, 19 Sep 2013 16:32:41 +0200 > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > > +extern void __get_online_cpus(void); > > > + > > > +static inline void get_online_cpus(void) > > > +{ > > > + might_sleep(); > > > + > > > + preempt_disable(); > > > + if (likely(!__cpuhp_writer || __cpuhp_writer == current)) > > > + this_cpu_inc(__cpuhp_refcount); > > > + else > > > + __get_online_cpus(); > > > + preempt_enable(); > > > +} > > > > > > This isn't much different than srcu_read_lock(). What about doing > > something like this: > > > > static inline void get_online_cpus(void) > > { > > might_sleep(); > > > > srcu_read_lock(&cpuhp_srcu); > > if (unlikely(__cpuhp_writer || __cpuhp_writer != current)) { > > srcu_read_unlock(&cpuhp_srcu); > > __get_online_cpus(); > > current->online_cpus_held++; > > } > > } > > There's a full memory barrier in srcu_read_lock(), while there was no > such thing in the previous fast path. Yeah, I mentioned this to Paul, and we talked about making srcu_read_lock() work with no mb's. But currently, doesn't get_online_cpus() just take a mutex? What's wrong with a mb() as it still kicks ass over what is currently there today? > > Also, why current->online_cpus_held()? That would make the write side > O(nr_tasks) instead of O(nr_cpus). ?? I'm not sure I understand this. The online_cpus_held++ was there for recursion. Can't get_online_cpus() nest? I was thinking it can. If so, once the "__cpuhp_writer" is set, we need to do __put_online_cpus() as many times as we did a __get_online_cpus(). I don't know where the O(nr_tasks) comes from. The ref here was just to account for doing the old "get_online_cpus" instead of a srcu_read_lock(). > > > static inline void put_online_cpus(void) > > { > > if (unlikely(current->online_cpus_held)) { > > current->online_cpus_held--; > > __put_online_cpus(); > > return; > > } > > > > srcu_read_unlock(&cpuhp_srcu); > > } > > Also, you might not have noticed but, srcu_read_{,un}lock() have an > extra idx thing to pass about. That doesn't fit with the hotplug api. I'll have to look a that, as I'm not exactly sure about the idx thing. > > > > > Then have the writer simply do: > > > > __cpuhp_write = current; > > synchronize_srcu(&cpuhp_srcu); > > > > <grab the mutex here> > > How does that do reader preference? Well, the point I was trying to do was to let readers go very fast (well, with a mb instead of a mutex), and then when the CPU hotplug happens, it goes back to the current method. That is, once we set __cpuhp_write, and then run synchronize_srcu(), the system will be in a state that does what it does today (grabbing mutexes, and upping refcounts). I thought the whole point was to speed up the get_online_cpus() when no hotplug is happening. This does that, and is rather simple. It only gets slow when hotplug is in effect. -- Steve -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>