On Tue, Nov 03, 2020 at 11:18:22AM -0800, Paul E. McKenney wrote:
> On Tue, Nov 03, 2020 at 05:33:50PM +0100, Uladzislau Rezki wrote:
> > On Tue, Nov 03, 2020 at 10:47:23AM -0500, Joel Fernandes wrote:
> > > On Thu, Oct 29, 2020 at 05:50:04PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > The current memmory-allocation interface presents to following
> > > > difficulties that this patch is designed to overcome:
> > > >
> > > > a) If built with CONFIG_PROVE_RAW_LOCK_NESTING, the lockdep will
> > > > complain about violation("BUG: Invalid wait context") of the
> > > > nesting rules. It does the raw_spinlock vs. spinlock nesting
> > > > checks, i.e. it is not legal to acquire a spinlock_t while
> > > > holding a raw_spinlock_t.
> > > >
> > > > Internally the kfree_rcu() uses raw_spinlock_t whereas the
> > > > "page allocator" internally deals with spinlock_t to access
> > > > to its zones. The code also can be broken from higher level
> > > > of view:
> > > > <snip>
> > > > raw_spin_lock(&some_lock);
> > > > kfree_rcu(some_pointer, some_field_offset);
> > > > <snip>
> > > >
> > > > b) If built with CONFIG_PREEMPT_RT. Please note, in that case spinlock_t
> > > > is converted into sleepable variant. Invoking the page allocator from
> > > > atomic contexts leads to "BUG: scheduling while atomic".
> > > >
> > > > c) call_rcu() is invoked from raw atomic context and kfree_rcu()
> > > > and kvfree_rcu() are expected to be called from atomic raw context
> > > > as well.
> > > >
> > > > Move out a page allocation from contexts which trigger kvfree_rcu()
> > > > function to the separate worker. When a k[v]free_rcu() per-cpu page
> > > > cache is empty a fallback mechanism is used and a special job is
> > > > scheduled to refill the per-cpu cache.
> > >
> > > Looks good, still reviewing here. BTW just for my education, I was
> > > wondering
> > > about Thomas's email:
> > > https://lkml.org/lkml/2020/8/11/939
> > >
> > > If slab allocations in pure raw-atomic context on RT is not allowed or
> > > recommended, should kfree_rcu() be allowed?
> > >
> > Thanks for reviewing, Joel :)
> >
> > The decision was made that we need to support kfree_rcu() from "real atomic
> > contexts",
> > to align with how it used to be before. We can go and just convert our
> > local locks
> > to the spinlock_t variant but that was not Paul goal, it can be that some
> > users need
> > kfree_rcu() for raw atomics.
>
> People invoke call_rcu() from raw atomics, and so we should provide
> the same for kfree_rcu(). Yes, people could work around a raw-atomic
> prohibition, but such prohibitions incur constant costs over time in
> terms of development effort, increased bug rate, and increased complexity.
> Yes, this does increase all of those for RCU, but the relative increase
> is negligible, RCU being what it is.
>
I see your point.
> > > slab can have same issue right? If per-cpu cache is drained, it has to
> > > allocate page from buddy allocator and there's no GFP flag to tell it
> > > about
> > > context where alloc is happening from.
> > >
> > Sounds like that. Apart of that, it might turn out soon that we or somebody
> > else will rise a question one more time about something GFP_RAW or
> > GFP_NOLOCKS.
> > So who knows..
>
> I would prefer that slab provide some way of dealing with raw atomic
> context, but the maintainers are thus far unconvinced.
>
I think, when preempt_rt is fully integrated to the kernel, we might get
new users with such demand. So, it is not a closed topic so far, IMHO.
> > > Or are we saying that we want to support kfree on RT from raw atomic
> > > atomic
> > > context, even though kmalloc is not supported? I hate to bring up this
> > > elephant in the room, but since I am a part of the people maintaining this
> > > code, I believe I would rather set some rules than supporting unsupported
> > > usages. :-\ (Once I know what is supported and what isn't that is). If
> > > indeed
> > > raw atomic kfree_rcu() is a bogus use case because of -RT, then we ought
> > > to
> > > put a giant warning than supporting it :-(.
> > >
> > We discussed it several times, the conclusion was that we need to support
> > kfree_rcu() from raw contexts. At least that was a clear signal from Paul
> > to me. I think, if we obtain the preemtable(), so it becomes versatile, we
> > can drop the patch that is in question later on in the future.
>
> Given a universally meaningful preemptible(), we could directly call
> the allocator in some cases. It might (or might not) still make sense
> to defer the allocation when preemptible() indicated that a direct call
> to the allocator was unsafe.
>
I do not have a strong opinion here. Giving the fact that maintaining of
such "deferring" is not considered as a big effort, i think, we can live
with it.
--
Vlad Rezki
