On Wed, Sep 22, 2021 at 12:53:50PM -0700, Paul E. McKenney wrote: > I wasn't saying that the patch doesn't work. But doesn't it add an IPI? > Or was I looking at it too early this morning? Ah, no. The patch allows a user-bound NOHZ_FULL task to be transitioned remotely. Unlike today, where they'll eventually poke it with a signal to force a kernel entry, which is bad m'kay :-) The code in question skips transitioning running tasks, seeing as you can't tell what they're doing etc.. Howver, with context tracking on you're supposedly able to tell they're in userspace without disturbing them -- except you really can't today. So if you can tell that a current running task is in userspace (hence my patch) you can allow the task to transition without any further ado, userspace is a safe state vs kernel text patching.
