Re: [PATCH] lib: fix compare_delta parameter order in percpu_counter_tree

[Mathieu Desnoyers]

On 2026-03-15 18:47, David CARLIER wrote:
> On Sun, 15 Mar 2026 at 22:00, Mathieu Desnoyers
[...]
> Any thought on how to extend the kunit
> > test to cover this ?
>
> Can something like this be applicable ?
>
> static void hpcc_test_compare_value_boundaries(struct kunit *test)
>    {
>        struct percpu_counter_tree pct;
>        struct percpu_counter_tree_level_item *counter_items;
>        unsigned long under = 0, over = 0;
>        int ret;
>
>        counter_items = kzalloc(percpu_counter_tree_items_size(), GFP_KERNEL);
>        KUNIT_ASSERT_PTR_NE(test, counter_items, NULL);
>        ret = percpu_counter_tree_init(&pct, counter_items, 32, GFP_KERNEL);
>        KUNIT_ASSERT_EQ(test, ret, 0);
>
>        percpu_counter_tree_set(&pct, 0);
>        percpu_counter_tree_approximate_accuracy_range(&pct, &under, &over);
> >        /* Skip if under == over (no gap to test) */
>        if (under == over)
>            goto out;

If we ever have over == under, then I think we could keep running this test,
but skip the "expect indeterminate" test items which depend on having
asymmetric ranges.

>        /* Positive boundary: v = under should be indeterminate (precise
> could be up to +under) */
>        KUNIT_EXPECT_EQ(test, 0,
>            percpu_counter_tree_approximate_compare_value(&pct, (long)under));
>
>        /* Positive boundary: v = over + 1 should be indeterminate */
>        KUNIT_EXPECT_EQ(test, 0,
>            percpu_counter_tree_approximate_compare_value(&pct,
> (long)(over + 1)));

This assumes that under > over, right ? In that case, I think we may
want to grab "under" and "over" values and copy them into a "a" and
"b" variables where a > b.

This way, the test does not cast on stone the library implementation
details: the fact that the "under" range is slightly larger than "over"
due to complement 2 +/- ranges.

Thanks!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com