Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Michal Hocko
2020-08-13T14:53:40Z

On Thu 13-08-20 16:34:57, Thomas Gleixner wrote:
> Michal Hocko <mhocko@suse.com> 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