On Wed, 23 Sep 2020 22:55:54 +0200 Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > Perhaps make migrate_disable() an anonymous local_lock()? > > > > This should lower the SHC in theory, if you can't have stacked migrate > > disables on the same CPU. > > I'm pretty sure this ends up in locking hell pretty fast and aside of > that it's not working for scenarios like: > > kmap_local(); > migrate_disable(); > ... > > copy_from_user() > -> #PF > -> schedule() > > which brought us into that discussion in the first place. You would stop > any other migrate disable user from running until the page fault is > resolved... Then scratch the idea of having anonymous local_lock() and just bring local_lock in directly? Then have a kmap local lock, which would only block those that need to do a kmap. Now as for migration disabled nesting, at least now we would have groupings of this, and perhaps the theorists can handle that. I mean, how is this much different that having a bunch of tasks blocked on a mutex with the owner is pinned on a CPU? migrate_disable() is a BKL of pinning affinity. If we only have local_lock() available (even on !RT), then it makes the blocking in groups. At least this way you could grep for all the different local_locks in the system and plug that into the algorithm for WCS, just like one would with a bunch of mutexes. -- Steve
