On Mon, Sep 23, 2013 at 11:13:03AM -0400, Steven Rostedt wrote: > 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. Well, for that the thing Oleg proposed works just fine and the preempt_disable() section vs synchronize_sched() is hardly magic. But I'd really like to get the writer pending case fast too. > 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). Still no point in using srcu for this; preempt_disable + synchronize_sched() is similar and much faster -- its the rcu_sched equivalent of what you propose. > 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. No, well, it also gets slow when a hotplug is pending, which can be quite a while if we go sprinkle get_online_cpus() all over the place and the machine is busy. One we start a hotplug attempt we must wait for all readers to quiesce -- since the lock is full reader preference this can take an infinite amount of time -- while we're waiting for this all 4k+ CPUs will be bouncing the one mutex around on every get_online_cpus(); of which we'll have many since that's the entire point of making them cheap, to use more of them. -- 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>