Re: [PATCH v7 04/21] slab: add sheaf support for batching kfree_rcu() operations

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Sep 09, 2025 at 11:08:20AM +0200, Uladzislau Rezki wrote:
> On Mon, Sep 08, 2025 at 02:45:11PM +0200, Vlastimil Babka wrote:
> > On 9/8/25 13:59, Uladzislau Rezki wrote:
> > > On Wed, Sep 03, 2025 at 02:59:46PM +0200, Vlastimil Babka wrote:
> > >> Extend the sheaf infrastructure for more efficient kfree_rcu() handling.
> > >> For caches with sheaves, on each cpu maintain a rcu_free sheaf in
> > >> addition to main and spare sheaves.
> > >> 
> > >> kfree_rcu() operations will try to put objects on this sheaf. Once full,
> > >> the sheaf is detached and submitted to call_rcu() with a handler that
> > >> will try to put it in the barn, or flush to slab pages using bulk free,
> > >> when the barn is full. Then a new empty sheaf must be obtained to put
> > >> more objects there.
> > >> 
> > >> It's possible that no free sheaves are available to use for a new
> > >> rcu_free sheaf, and the allocation in kfree_rcu() context can only use
> > >> GFP_NOWAIT and thus may fail. In that case, fall back to the existing
> > >> kfree_rcu() implementation.
> > >> 
> > >> Expected advantages:
> > >> - batching the kfree_rcu() operations, that could eventually replace the
> > >>   existing batching
> > >> - sheaves can be reused for allocations via barn instead of being
> > >>   flushed to slabs, which is more efficient
> > >>   - this includes cases where only some cpus are allowed to process rcu
> > >>     callbacks (Android)
> > >> 
> > >> Possible disadvantage:
> > >> - objects might be waiting for more than their grace period (it is
> > >>   determined by the last object freed into the sheaf), increasing memory
> > >>   usage - but the existing batching does that too.
> > >> 
> > >> Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny
> > >> implementation favors smaller memory footprint over performance.
> > >> 
> > >> Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to
> > >> count how many kfree_rcu() used the rcu_free sheaf successfully and how
> > >> many had to fall back to the existing implementation.
> > >> 
> > >> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> > >> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> > >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > >> ---
> > >>  mm/slab.h        |   2 +
> > >>  mm/slab_common.c |  24 +++++++
> > >>  mm/slub.c        | 192 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> > >>  3 files changed, 216 insertions(+), 2 deletions(-)
> > >> 
> > >> diff --git a/mm/slab.h b/mm/slab.h
> > >> index 206987ce44a4d053ebe3b5e50784d2dd23822cd1..f1866f2d9b211bb0d7f24644b80ef4b50a7c3d24 100644
> > >> --- a/mm/slab.h
> > >> +++ b/mm/slab.h
> > >> @@ -435,6 +435,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
> > >>  	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
> > >>  }
> > >>  
> > >> +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
> > >> +
> > >>  #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
> > >>  			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
> > >>  			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \
> > >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> > >> index e2b197e47866c30acdbd1fee4159f262a751c5a7..2d806e02568532a1000fd3912db6978e945dcfa8 100644
> > >> --- a/mm/slab_common.c
> > >> +++ b/mm/slab_common.c
> > >> @@ -1608,6 +1608,27 @@ static void kfree_rcu_work(struct work_struct *work)
> > >>  		kvfree_rcu_list(head);
> > >>  }
> > >>  
> > >> +static bool kfree_rcu_sheaf(void *obj)
> > >> +{
> > >> +	struct kmem_cache *s;
> > >> +	struct folio *folio;
> > >> +	struct slab *slab;
> > >> +
> > >> +	if (is_vmalloc_addr(obj))
> > >> +		return false;
> > >> +
> > >> +	folio = virt_to_folio(obj);
> > >> +	if (unlikely(!folio_test_slab(folio)))
> > >> +		return false;
> > >> +
> > >> +	slab = folio_slab(folio);
> > >> +	s = slab->slab_cache;
> > >> +	if (s->cpu_sheaves)
> > >> +		return __kfree_rcu_sheaf(s, obj);
> > >> +
> > >> +	return false;
> > >> +}
> > >> +
> > >>  static bool
> > >>  need_offload_krc(struct kfree_rcu_cpu *krcp)
> > >>  {
> > >> @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
> > >>  	if (!head)
> > >>  		might_sleep();
> > >>  
> > >> +	if (kfree_rcu_sheaf(ptr))
> > >> +		return;
> > >> +
> > > Uh.. I have some concerns about this.
> > > 
> > > This patch introduces a new path which is a collision to the
> > > existing kvfree_rcu() logic. It implements some batching which
> > > we already have.
> > 
> > Yes but for caches with sheaves it's better to recycle the whole sheaf (as
> > described), which is so different from the existing batching scheme that I'm
> > not sure if there's a sensible way to combine them.
> > 
> > > - kvfree_rcu_barrier() does not know about "sheaf" path. Am i missing
> > >   something? How do you guarantee that kvfree_rcu_barrier() flushes
> > >   sheafs? If it is part of kvfree_rcu() it has to care about this.
> > 
> > Hm good point, thanks. I've taken care of handling flushing related to
> > kfree_rcu() sheaves in kmem_cache_destroy(), but forgot that
> > kvfree_rcu_barrier() can be also used outside of that - we have one user in
> > codetag_unload_module() currently.
> > 
> > > - we do not allocate in kvfree_rcu() path because of PREEMMPT_RT, i.e.
> > >   kvfree_rcu() is supposed it can be called from the non-sleeping contexts.
> > 
> > Hm I could not find where that distinction is in the code, can you give a
> > hint please. In __kfree_rcu_sheaf() I do only have a GFP_NOWAIT attempt.
> > 
> For PREEMPT_RT a regular spin-lock is an rt-mutex which can sleep. We
> made kvfree_rcu() to make it possible to invoke it from non-sleep contexts:
> 
> CONFIG_PREEMPT_RT
> 
> preempt_disable() or something similar;
>  kvfree_rcu();
>   GFP_NOWAIT - lock rt-mutex
> 
> If GFP_NOWAIT semantic does not access any spin-locks then we are safe
> or if it uses raw_spin_locks.
> 
And this is valid only for double argument, single argument you can
invoke from sleeping context only, then you can allocate.

--
Uladzislau Rezki