On Fri, May 08, 2020 at 11:34:07PM +0200, Joerg Roedel wrote: > Hi Peter, > > thanks for reviewing this! > > On Fri, May 08, 2020 at 09:20:00PM +0200, Peter Zijlstra wrote: > > The only concern I have is the pgd_lock lock hold times. > > > > By not doing on-demand faults anymore, and consistently calling > > sync_global_*(), we iterate that pgd_list thing much more often than > > before if I'm not mistaken. > > Should not be a problem, from what I have seen this function is not > called often on x86-64. The vmalloc area needs to be synchronized at > the top-level there, which is by now P4D (with 4-level paging). And the > vmalloc area takes 64 entries, when all of them are populated the > function will not be called again. Right; it's just that the moment you do trigger it, it'll iterate that pgd_list and that is potentially huge. Then again, that's not a new problem. I suppose we can deal with it if/when it becomes an actual problem. I had a quick look and I think it might be possible to make it an RCU managed list. We'd have to remove the pgd_list entry in put_task_struct_rcu_user(). Then we can play games in sync_global_*() and use RCU iteration. New tasks (which can be missed in the RCU iteration) will already have a full clone of the PGD anyway. But like said, something for later I suppose.
