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

Paul E. McKenney
2020-06-18T17:32:09Z

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?

> Paul, thank you to take it in!

Thank you for persisting!

Thanx, Paul