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... Yeah, so I don't think that's actually a problem. The premise is that *IFF* NOHZ_FULL stays in userspace, then it will never observe the IPI. If it violates this by doing syscalls or other kernel entries; it gets to keep the pieces.
