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

[Liam R. Howlett]

* Uladzislau Rezki <urezki at 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?

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.

> 
> 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?

Thanks,
Liam