Subject: Re: [RFC PATCH 0/7] Reviving the slab destructor to tackle the percpu allocator scalability problem

From: Harry Yoo <[email-masked]>
Date: Mon, 28 Apr 2025 10:18:34 +0900

On Fri, Apr 25, 2025 at 11:42:27AM +0100, Pedro Falcato wrote:
> On Fri, Apr 25, 2025 at 07:12:02PM +0900, Harry Yoo wrote:
> > On Thu, Apr 24, 2025 at 12:28:37PM +0100, Pedro Falcato wrote:
> > > On Thu, Apr 24, 2025 at 05:07:48PM +0900, Harry Yoo wrote:
> > > > Overview
> > > > ========
> > > > 
> > > > The slab destructor feature existed in early days of slab allocator(s).
> > > > It was removed by the commit c59def9f222d ("Slab allocators: Drop support
> > > > for destructors") in 2007 due to lack of serious use cases at that time.
> > > > 
> > > > Eighteen years later, Mateusz Guzik proposed [1] re-introducing a slab
> > > > constructor/destructor pair to mitigate the global serialization point
> > > > (pcpu_alloc_mutex) that occurs when each slab object allocates and frees
> > > > percpu memory during its lifetime.
> > > > 
> > > > Consider mm_struct: it allocates two percpu regions (mm_cid and rss_stat),
> > > > so each allocate–free cycle requires two expensive acquire/release on
> > > > that mutex.
> > > > 
> > > > We can mitigate this contention by retaining the percpu regions after
> > > > the object is freed and releasing them only when the backing slab pages
> > > > are freed.
> > > > 
> > > > How to do this with slab constructors and destructors: the constructor
> > > > allocates percpu memory, and the destructor frees it when the slab pages
> > > > are reclaimed; this slightly alters the constructor’s semantics,
> > > > as it can now fail.
> > > > 
> > > 
> > > I really really really really don't like this. We're opening a pandora's box
> > > of locking issues for slab deadlocks and other subtle issues. IMO the best
> > > solution there would be, what, failing dtors? which says a lot about the whole
> > > situation...
> > > 
> > > Case in point:
> > 
> > <...snip...>
> > 
> > > Then there are obviously other problems like: whatever you're calling must
> > > not ever require the slab allocator (directly or indirectly) and must not
> > > do direct reclaim (ever!), at the risk of a deadlock. The pcpu allocator
> > > is a no-go (AIUI!) already because of such issues.
> > 
> > Could you please elaborate more on this?
>
> Well, as discussed multiple-times both on-and-off-list, the pcpu allocator is
> not a problem here because the freeing path takes a spinlock, not a mutex.

Yes, and it seems to be a leaf spinlock (no code path in the kernel takes
any other locks while holding the lock).

> But obviously you can see the fun locking horror dependency chains
> we're creating with this patchset.
>
> ->ctor() needs to be super careful calling things, avoiding
> any sort of loop.

You mean recursion to avoid exhausting the kernel stack?

It'd be fine as long as ->ctor does not allocate objects from
the same cache (and of course it should not).

> ->dtor() needs to be super careful calling things, avoiding
> _any_ sort of direct reclaim possibilities.

Why would a dtor _ever_ need to perform direct reclamation?

>You also now need to pass a gfp_t  to both ->ctor and ->dtor.

Passing gfp_t to ->ctor agreed, but why to ->dtor?

Destructors should not allocate any memory and thus no need for
'Get Free Page' flags.

> So basically:
> - ->ctor takes more args, can fail, can do fancier things (multiple allocations,
>   lock holding, etc, can be hidden with a normal kmem_cache_alloc;

Speaking of deadlocks involving ->ctor, of course, you shouldn't allocate
mm_struct while holding pcpu_lock, or allocate a pgd while holding pgd_lock.
I don't think avoiding deadlocks caused by ->ctor is that difficult, and
lockdep can detect them even if someone makes a mistake.

> - ->dtor *will* do fancy things like recursing back onto the slab allocator and
>   grabbing locks

AIUI it can't recurse back onto the slab allocator.
'Taking only leaf spinlocks' is a very restrictive rule.

For example, slab takes list_lock, and it is not a leaf spinlock because
in some path slab can take other locks while holding it. And thus you can't
call kmem_cache_free() directly in ->dtor.

'Doing fancy things and grabbing locks' in dtor is safe as long as
you 1) disable interrupts and 2) take only leaf spinlocks.
 
> - a normal kmem_cache_free can suddenly attempt to grab !SLUB locks as it tries
>   to dispose of slabs. It can also uncontrollably do $whatever.

'Can uncontrollably do $whatever' is not true.
It's safe as long as ->dtor only takes leaf spinlocks.

> - a normal kmem_cache_alloc can call vast swaths of code, uncontrollably, due to
>   ->ctor. It can also set off direct reclaim, and thus run into all sorts of kmem_
>   cache_free/slab disposal issues
> 
> - a normal, possibly-unrelated GFP_KERNEL allocation can also run into all of these
>   issues by purely starting up shrinkers on direct reclaim as well.

I don't see that is a problem (as long as ->dtor only takes leaf spinlocks).

-- 
Cheers,
Harry / Hyeonggon