On Mon, 19 Aug 2013, Ming Lei wrote: > On Mon, Aug 19, 2013 at 10:42 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 19 Aug 2013, Ming Lei wrote: > > > >> Because usb_hcd_submit_urb is in the hotest path of usb core, > >> so use percpu counter to count URB instead of using atomic variable > >> because atomic operations are much slower than percpu operations. > > > > This seems like a ridiculous amount of additional overhead for a simple > > counter. The kernel doesn't even use this value for anything; it's > > only purpose is to allow userspace to see how many URBs have been > > transferred for a device. (I don't know what programs use this > > information. Powertop maybe?) > > That is why I want to remove the expensive atomic inc/dec, or can we > remove the counter? No doubt somebody would complain if the counter was removed. Who added it in the first place, and for what reason? > > Do you have any reason to believe this will really improve performance > > at all? > > Please see my reply on Greg's comments. As far as I can see, this counter does not need to be exact. Why not simply make it a non-atomic unsigned int? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
