Re: [PATCH v2 09/16] rcu/tree: Maintain separate array for vmalloc ptrs

Uladzislau Rezki
2020-06-18T17:35:27Z

> >
> > I don't think that replacing direct function calls with indirect function
> > calls is a great suggestion with the current state of play around branch
> > prediction.
> >
> > I'd suggest:
> >
> > rcu_lock_acquire(&rcu_callback_map);
> > trace_rcu_invoke_kfree_bulk_callback(rcu_state.name,
> > bkvhead[i]->nr_records, bkvhead[i]->records);
> > if (i == 0) {
> > kfree_bulk(bkvhead[i]->nr_records,
> > bkvhead[i]->records);
> > } else {
> > for (j = 0; j < bkvhead[i]->nr_records; j++) {
> > vfree(bkvhead[i]->records[j]);
> > }
> > }
> > rcu_lock_release(&rcu_callback_map);
> >
> > But I'd also suggest a vfree_bulk be added. There are a few things
> > which would be better done in bulk as part of the vfree process
> > (we batch them up already, but i'm sure we could do better).
>
> I suspect that he would like to keep the tracing.
>
> It might be worth trying the branches, given that they would be constant
> and indexed by "i". The compiler might well remove the indirection.
>
> The compiler guys brag about doing so, which of course might or might
> not have any correlation to a given compiler actually doing so. :-/
>
> Having a vfree_bulk() might well be useful, but I would feel more
> confidence in that if there were other callers of kfree_bulk().
>
Hmm... I think replacing that with vfree_bulk() is a good idea though.

>
> But again, either way, future work as far as this series is concerned.
>
What do you mean: is concerned?

We are planning to implement kfree_rcu() to be integrated directly into
SLAB: SLAB, SLUB, SLOB. So, there are plenty of future work :)

--
Vlad Rezki