Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree_rcu() operations

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Sep 10, 2025 at 10:01:06AM +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.
> 
> Also for now skip the usage of rcu sheaf for CONFIG_PREEMPT_RT as the
> contexts where kfree_rcu() is called might not be compatible with taking
> a barn spinlock or a GFP_NOWAIT allocation of a new sheaf taking a
> spinlock - the current kfree_rcu() implementation avoids doing that.
> 
> Teach kvfree_rcu_barrier() to flush all rcu_free sheaves from all caches
> that have them. This is not a cheap operation, but the barrier usage is
> rare - currently kmem_cache_destroy() or on module unload.
> 
> 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.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slab.h        |   3 +
>  mm/slab_common.c |  26 ++++++
>  mm/slub.c        | 266 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 293 insertions(+), 2 deletions(-)
> 
> @@ -3840,6 +3895,80 @@ static void flush_all(struct kmem_cache *s)
>  	cpus_read_unlock();
>  }
>  
> +/* needed for kvfree_rcu_barrier() */
> +void flush_all_rcu_sheaves()
> +{
> +	struct slub_percpu_sheaves *pcs;
> +	struct slub_flush_work *sfw;
> +	struct kmem_cache *s;
> +	bool flushed = false;
> +	unsigned int cpu;
> +
> +	cpus_read_lock();
> +	mutex_lock(&slab_mutex);
> +
> +	list_for_each_entry(s, &slab_caches, list) {
> +		if (!s->cpu_sheaves)
> +			continue;
> +
> +		mutex_lock(&flush_lock);
> +
> +		for_each_online_cpu(cpu) {
> +			sfw = &per_cpu(slub_flush, cpu);
> +			pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
> +
> +			if (!pcs->rcu_free || !pcs->rcu_free->size) {

Is the compiler allowed to compile this to read pcs->rcu_free twice?
Something like:

flush_all_rcu_sheaves()			__kfree_rcu_sheaf()

pcs->rcu_free != NULL
					pcs->rcu_free = NULL
pcs->rcu_free == NULL
/* NULL-pointer-deref */
pcs->rcu_free->size

> +				sfw->skip = true;
> +				continue;
> +			}
>
> +			INIT_WORK(&sfw->work, flush_rcu_sheaf);
> +			sfw->skip = false;
> +			sfw->s = s;
> +			queue_work_on(cpu, flushwq, &sfw->work);
> +			flushed = true;
> +		}
> +
> +		for_each_online_cpu(cpu) {
> +			sfw = &per_cpu(slub_flush, cpu);
> +			if (sfw->skip)
> +				continue;
> +			flush_work(&sfw->work);
> +		}
> +
> +		mutex_unlock(&flush_lock);
> +	}
> +
> +	mutex_unlock(&slab_mutex);
> +	cpus_read_unlock();
> +
> +	if (flushed)
> +		rcu_barrier();

I think we need to call rcu_barrier() even if flushed == false?

Maybe a kvfree_rcu()'d object was already waiting for the rcu callback to
be processed before flush_all_rcu_sheaves() is called, and
in flush_all_rcu_sheaves() we skipped all (cache, cpu) pairs,
so flushed == false but the rcu callback isn't processed yet
by the end of the function?

That sounds like a very unlikely to happen in a realistic scenario,
but still possible...

-- 
Cheers,
Harry / Hyeonggon