> On Thu 13-08-20 08:41:59, Paul E. McKenney wrote: > > On Thu, Aug 13, 2020 at 04:53:35PM +0200, Michal Hocko wrote: > > > 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. > > > > That is a reasonable high-level summary. > > > > > 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. > > > > Unless I am missing something subtle, the problem with this approach > > is that in production-environment CONFIG_PREEMPT_NONE=y kernels, there > > is no way at runtime to distinguish between holding a spinlock on the > > one hand and holding a raw spinlock on the other. Therefore, without > > some sort of indication from the caller, this approach will not make > > CONFIG_PREEMPT_NONE=y users happy. > > If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity > check then there is no functional problem - GFP_RT_SAFE would still be > GFP_NOWAIT so functional wise the allocator will still do the right > thing. > > [...] > > > > 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. > > > > Exactly, and avoiding changing this semantic for current users is > > precisely why we are proposing some sort of indication to be passed > > into the allocation request. In Uladzislau's patch, this was the > > __GFP_NO_LOCKS flag, but whatever works. > > As I've tried to explain already, I would really hope we can do without > any new gfp flags. We are running out of them and they tend to generate > a lot of maintenance burden. There is a lot of abuse etc. We should also > not expose such an implementation detail of the allocator to callers > because that would make future changes even harder. The alias, on the > othere hand already builds on top of existing NOWAIT semantic and it > just helps the allocator to complain about a wrong usage while it > doesn't expose any internals. > I know that Matthew and me raised it. We do can handle it without introducing any flag. I mean just use 0 as argument to the page_alloc(gfp_flags = 0) i.e. #define __GFP_NO_LOCKS 0 so it will be handled same way how it is done in the "mm: Add __GFP_NO_LOCKS flag" I can re-spin the RFC patch and send it out for better understanding. Does it work for you, Michal? Or it is better just to drop the patch here? -- Vlad Rezki
