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 10:35:15AM -0400, Liam R. Howlett wrote:
> * Uladzislau Rezki <urezki@gmail.com> [250909 05:08]:
> 
> ...
> 
> > 
> > > > - call_rcu() can be slow, therefore we do not use it in the kvfree_rcu().
> > > 
> > > If call_rcu() is called once per 32 kfree_rcu() filling up the rcu sheaf, is
> > > it still too slow?
> > >
> > You do not know where in a queue this callback lands, in the beginning,
> > in the end, etc. It is part of generic list which is processed one by
> > one. It can contain thousands of callbacks.
> 
> How does this differ from kvfree_rcu()?
> 
> Surely if you have enough calls to kvfree_rcu(), you will end up with a
> large list of frees before the end of a grace period?  Our placement in
> the freeing order would still be dependent on what else is using the
> infrastructure in the same grace period, right?
> 
In kfree_rcu() we use page blocks to carry pointers. Lists can be used
if there is a low memory condition so a page can not be allocated or
cache is empty. But this is not part of carr_rcu() track in any way.

Right regular call_rcu() puts callback into its own internal lists and
they are processed one by one during list iteration. In such lists you
can have hundred of thousand callback.

>
> How is kvfree_rcu() affected by rcu callback offloading to a specific
> cpu and rcu expedite?  Often these two features come into play for
> certain workloads which are of concern to us.
> 
We maintain a separate path. Offload is done after a grace period is
over. It is classic way. Historically all deferred freeing was one
call_rcu() per ptr.

> > 
> > If performance is not needed then it is not an issue. But in
> > kvfree_rcu() we do not use it, because of we want to offload
> > fast.
> 
> Today, I free things using call_rcu() and a custom callback so I would
> think stacking 32 together would make the list shorter, but latency
> would increase by waiting until there are 32.
> 
> If we wanted to flush the kvfree_rcu() list, is it done in the same way
> as the call_rcu() list, or is there a better way?
> 
For this case we have kvfree_rcu_barrier(). It is not same as call_rcu()
flushing.

--
Uladzislau Rezki