On Wed, 2012-09-26 at 15:39 +0200, Andrew Jones wrote: > On Wed, Sep 26, 2012 at 03:26:11PM +0200, Peter Zijlstra wrote: > > On Wed, 2012-09-26 at 15:20 +0200, Andrew Jones wrote: > > > Wouldn't a clean solution be to promote a task's scheduler > > > class to the spinner class when we PLE (or come from some special > > > syscall > > > for userspace spinlocks?)? > > > > Userspace spinlocks are typically employed to avoid syscalls.. > > I'm guessing there could be a slow path - spin N times and then give > up and yield. Much better they should do a blocking futex call or so, once you do the syscall you're in kernel space anyway and have paid the transition cost. > > > > > That class would be higher priority than the > > > fair class and would schedule in FIFO order, but it would only run its > > > tasks for short periods before switching. > > > > Since lock hold times aren't limited, esp. for things like userspace > > 'spin' locks, you've got a very good denial of service / opportunity for > > abuse right there. > > Maybe add some throttling to avoid overuse/maliciousness? At which point you're pretty much back to where you started. A much better approach is using things like priority inheritance, which can be extended to cover the fair class just fine.. Also note that user-space spinning is inherently prone to live-locks when combined with the static priority RT scheduling classes. In general its a very bad idea.. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
