Re: [PATCH 01/16] rcu/tree: Add a work to allocate pages from regular context

[Paul E. McKenney]

On Wed, Nov 04, 2020 at 01:35:53PM +0100, Uladzislau Rezki wrote:
> 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.

Agreed!  ;-)

> > > > 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.

And agreed here as well.  If this were instead a large body of complex
code, I might feel otherwise.  But as it is, why worry?

							Thanx, Paul