Re: [PATCH v3 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Aug 14, 2019 at 10:38:17AM -0400, Joel Fernandes wrote:
> On Tue, Aug 13, 2019 at 12:07:38PM -0700, Paul E. McKenney wrote:
 [snip]
> > > - * Queue an RCU callback for lazy invocation after a grace period.
> > > - * This will likely be later named something like "call_rcu_lazy()",
> > > - * but this change will require some way of tagging the lazy RCU
> > > - * callbacks in the list of pending callbacks. Until then, this
> > > - * function may only be called from __kfree_rcu().
> > > + * Maximum number of kfree(s) to batch, if this limit is hit then the batch of
> > > + * kfree(s) is queued for freeing after a grace period, right away.
> > >   */
> > > -void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > > +struct kfree_rcu_cpu {
> > > +	/* The rcu_work node for queuing work with queue_rcu_work(). The work
> > > +	 * is done after a grace period.
> > > +	 */
> > > +	struct rcu_work rcu_work;
> > > +
> > > +	/* The list of objects being queued in a batch but are not yet
> > > +	 * scheduled to be freed.
> > > +	 */
> > > +	struct rcu_head *head;
> > > +
> > > +	/* The list of objects that have now left ->head and are queued for
> > > +	 * freeing after a grace period.
> > > +	 */
> > > +	struct rcu_head *head_free;
> > 
> > So this is not yet the one that does multiple batches concurrently
> > awaiting grace periods, correct?  Or am I missing something subtle?
> 
> Yes, it is not. I honestly, still did not understand that idea. Or how it
> would improve things. May be we can discuss at LPC on pen and paper? But I
> think that can also be a follow-up optimization.

I got it now. Basically we can benefit a bit more by having another list
(that is have multiple kfree_rcu batches in flight). I will think more about
it - but hopefully we don't need to gate this patch by that.

It'll be interesting to see what rcuperf says about such an improvement :)

thanks,

 - Joel