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

Paul E. McKenney
2019-08-14T18:44:38Z

On Wed, Aug 14, 2019 at 01:22:33PM -0400, Joel Fernandes wrote:
> 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.

I am willing to take this as a later optimization.

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

Indeed, no guarantees either way. The reason for hope assumes a busy
system where each grace period is immediately followed by another
grace period. On such a system, the current setup allows each CPU to
make use only of every second grace period for its kfree_rcu() work.
The hope would therefore be that this would reduce the memory footprint
substantially with no increase in overhead.

But no way to know without trying it! ;-)

Thanx, Paul