On Wed, Apr 05, 2023 at 09:52:26PM +0200, Peter Zijlstra wrote: > On Wed, Apr 05, 2023 at 04:45:32PM -0300, Marcelo Tosatti wrote: > > On Wed, Apr 05, 2023 at 01:10:07PM +0200, Frederic Weisbecker wrote: > > > On Wed, Apr 05, 2023 at 12:44:04PM +0200, Frederic Weisbecker wrote: > > > > On Tue, Apr 04, 2023 at 04:42:24PM +0300, Yair Podemsky wrote: > > > > > + int state = atomic_read(&ct->state); > > > > > + /* will return true only for cpus in kernel space */ > > > > > + return state & CT_STATE_MASK == CONTEXT_KERNEL; > > > > > +} > > > > > > > > Also note that this doesn't stricly prevent userspace from being interrupted. > > > > You may well observe the CPU in kernel but it may receive the IPI later after > > > > switching to userspace. > > > > > > > > We could arrange for avoiding that with marking ct->state with a pending work bit > > > > to flush upon user entry/exit but that's a bit more overhead so I first need to > > > > know about your expectations here, ie: can you tolerate such an occasional > > > > interruption or not? > > > > > > Bah, actually what can we do to prevent from that racy IPI? Not much I fear... > > > > Use a different mechanism other than an IPI to ensure in progress > > __get_free_pages_fast() has finished execution. > > > > Isnt this codepath slow path enough that it can use > > synchronize_rcu_expedited? > > To actually hit this path you're doing something really dodgy. Apparently khugepaged is using the same infrastructure: $ grep tlb_remove_table khugepaged.c tlb_remove_table_sync_one(); tlb_remove_table_sync_one(); So just enabling khugepaged will hit that path.
