On Thu, Jan 25, 2018 at 09:58:20AM +0100, Peter Zijlstra wrote: > On Wed, Jan 24, 2018 at 04:36:41PM -0800, Tim Chen wrote: > > These two patches provide optimization to skip IBPB for this > > commonly encountered scenario: > > We could switch to a kernel idle thread and then back to the original > > process such as: > > process A -> idle -> process A > > > > In such scenario, we do not have to do IBPB here even though the process > > is non-dumpable, as we are switching back to the same process after > > an hiatus. > > > > The cost is to have an extra pointer to track the last mm we were using > > before > > switching to the init_mm used by idle. But avoiding the extra IBPB > > is probably worth the extra memory for such a common scenario. > > So we already track active_mm for kernel threads. I can't immediately > see where this fails for idle and your changelog doesn't say.
idle_task_exit() explicitly switches back to init_mm when we take the CPU offline, this very much suggests the active_mm thing works for idle too. This means that 'A -> idle -> A' should never pass through switch_mm to begin with. Please clarify how you think it does.
