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

Uladzislau Rezki
2020-06-18T17:56:29Z

On Thu, Jun 18, 2020 at 10:32:06AM -0700, Paul E. McKenney wrote:
> On Thu, Jun 18, 2020 at 07:25:04PM +0200, Uladzislau Rezki wrote:
> > > > + // Handle two first channels.
> > > > + for (i = 0; i < FREE_N_CHANNELS; i++) {
> > > > + for (; bkvhead[i]; bkvhead[i] = bnext) {
> > > > + bnext = bkvhead[i]->next;
> > > > + debug_rcu_bhead_unqueue(bkvhead[i]);
> > > > +
> > > > + rcu_lock_acquire(&rcu_callback_map);
> > > > + if (i == 0) { // kmalloc() / kfree().
> > > > + trace_rcu_invoke_kfree_bulk_callback(
> > > > + rcu_state.name, bkvhead[i]->nr_records,
> > > > + bkvhead[i]->records);
> > > > +
> > > > + kfree_bulk(bkvhead[i]->nr_records,
> > > > + bkvhead[i]->records);
> > > > + } else { // vmalloc() / vfree().
> > > > + for (j = 0; j < bkvhead[i]->nr_records; j++) {
> > > > + trace_rcu_invoke_kfree_callback(
> > > > + rcu_state.name,
> > > > + bkvhead[i]->records[j], 0);
> > > > +
> > > > + vfree(bkvhead[i]->records[j]);
> > > > + }
> > > > + }
> > > > + rcu_lock_release(&rcu_callback_map);
> > >
> > > Not an emergency, but did you look into replacing this "if" statement
> > > with an array of pointers to functions implementing the legs of the
> > > "if" statement? If nothing else, this would greatly reduced indentation.
> > >
> > >
> > > I am taking this as is, but if you have not already done so, could you
> > > please look into this for a follow-up patch?
> > >
> > I do not think it makes sense, because it would require to check each
> > pointer in the array, what can lead to many branching, i.e. "if-else"
> > instructions.
>
> Mightn't the compiler simply unroll the outer loop? Then the first
> unrolled iteration of that loop would contain the then-clause and
> the second unrolled iteration would contain the else-clause. At that
> point, there would be no checking, just direct calls.
>
> Or am I missing something?
>
If we mix pointers, then we can do free per pointer only. I mean in that
case we will not be able to use kfree_bulk() interface for freeing SLAB
memory and the code would converted to something like:

<snip>
while (nr_objects_in_array > 0) {
if (is_vmalloc_addr(array[X]))
vfree(array[X]);
else
kfree(array[X]);
}
<snip>

> > Paul, thank you to take it in!
>
> Thank you for persisting!
>
Welcome :)

--
Vlad Rezki