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

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

On Fri, Oct 31, 2025 at 10:32:54PM +0100, Daniel Gomez wrote:
> 
> 
> On 10/09/2025 10.01, 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>
> 
> Hi Vlastimil,
> 
> This patch increases kmod selftest (stress module loader) runtime by about
> ~50-60%, from ~200s to ~300s total execution time. My tested kernel has
> CONFIG_KVFREE_RCU_BATCHED enabled. Any idea or suggestions on what might be
> causing this, or how to address it?

This is likely due to increased kvfree_rcu_barrier() during module unload.

It currently iterates over all CPUs x slab caches (that enabled sheaves,
there should be only a few now) pair to make sure rcu sheaf is flushed
by the time kvfree_rcu_barrier() returns.

Just being curious, do you have any serious workload that depends on
the performance of module unload?

-- 
Cheers,
Harry / Hyeonggon