On Thu 13-08-20 16:34:57, Thomas Gleixner wrote: > Michal Hocko <mhocko@xxxxxxxx> writes: > > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote: > >> It basically requires to convert the wait queue to something else. Is > >> the waitqueue strict single waiter? > > > > I would have to double check. From what I remember only kswapd should > > ever sleep on it. > > That would make it trivial as we could simply switch it over to rcu_wait. > > >> So that should be: > >> > >> if (!preemptible() && gfp == GFP_RT_NOWAIT) > >> > >> which is limiting the damage to those callers which hand in > >> GFP_RT_NOWAIT. > >> > >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits > >> zone->lock in the wrong context. And we want to know about that so we > >> can look at the caller and figure out how to solve it. > > > > Yes, that would have to somehow need to annotate the zone_lock to be ok > > in those paths so that lockdep doesn't complain. > > That opens the worst of all cans of worms. If we start this here then > Joe programmer and his dog will use these lockdep annotation to evade > warnings and when exposed to RT it will fall apart in pieces. Just that > at that point Joe programmer moved on to something else and the usual > suspects can mop up the pieces. We've seen that all over the place and > some people even disable lockdep temporarily because annotations don't > help. Hmm. I am likely missing something really important here. We have two problems at hand: 1) RT will become broken as soon as this new RCU functionality which requires an allocation from inside of raw_spinlock hits the RT tree 2) lockdep splats which are telling us that early because of the raw_spinlock-> spin_lock dependency. 1) can be handled by handled by the bailing out whenever we have to use zone->lock inside the buddy allocator - essentially even more strict NOWAIT semantic than we have for RT tree - proposed (pseudo) patch is trying to describe that. 2) would become a false positive if 1) is in place, right? RT wouldn't do the illegal nesting and !RT would just work fine because GFP_RT_NOWAIT would be simply GFP_NOWAIT & ~__GFP_KSWAPD_RECLAIM. Why should we limit the functionality of the allocator for something that is not a real problem? > PeterZ might have opinions about that too I suspect. > > Really, if your primary lockless caches are empty then any allocation > which comes from deep atomic context should simply always fail. Being > stuck in an interrupt handler or even deeper for 200+ microseconds > waiting for zone lock is just bonkers IMO. That would require changing NOWAIT/ATOMIC allocations semantic quite drastically for !RT kernels as well. I am not sure this is something we can do. Or maybe I am just missing your point. -- Michal Hocko SUSE Labs
