Re: [PATCH v7 2/5] bug/kunit: Reduce runtime impact of warning backtrace suppression

Peter Zijlstra
2026-04-20T14:45:02Z

On Mon, Apr 20, 2026 at 02:28:04PM +0200, Albert Esteve wrote:
> From: Alessandro Carminati <acarmina@redhat.com>
>
> KUnit support is not consistently present across distributions, some
> include it in their stock kernels, while others do not.
> While both KUNIT and KUNIT_SUPPRESS_BACKTRACE can be considered debug
> features, the fact that some distros ship with KUnit enabled means it's
> important to minimize the runtime impact of this patch.
>
> To that end, this patch adds an atomic counter that tracks the number
> of active suppressions. __kunit_is_suppressed_warning() checks this
> counter first and returns immediately when no suppressions are active,
> avoiding RCU-protected list traversal in the common case.
>
> Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
> lib/kunit/bug.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
> index 356c8a5928828..a7a88f0670d44 100644
> --- a/lib/kunit/bug.c
> +++ b/lib/kunit/bug.c
> @@ -8,6 +8,7 @@
>
> #include <kunit/bug.h>
> #include <kunit/resource.h>
> +#include <linux/atomic.h>
> #include <linux/export.h>
> #include <linux/rculist.h>
> #include <linux/sched.h>
> @@ -15,11 +16,13 @@
> #ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
>
> static LIST_HEAD(suppressed_warnings);
> +static atomic_t suppressed_warnings_cnt = ATOMIC_INIT(0);
>
> static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
> {
> list_del_rcu(&warning->node);
> synchronize_rcu(); /* Wait for readers to finish */
> + atomic_dec(&suppressed_warnings_cnt);
> }
>
> KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
> @@ -37,6 +40,7 @@ __kunit_start_suppress_warning(struct kunit *test)
> return NULL;
>
> warning->task = current;
> + atomic_inc(&suppressed_warnings_cnt);
> list_add_rcu(&warning->node, &suppressed_warnings);
>
> ret = kunit_add_action_or_reset(test,
> @@ -68,6 +72,9 @@ bool __kunit_is_suppressed_warning(void)
> {
> struct __suppressed_warning *warning;
>
> + if (!atomic_read(&suppressed_warnings_cnt))
> + return false;
> +
> rcu_read_lock();
> list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> if (warning->task == current) {
>

So the thing you're skipping is:

rcu_read_lock();
list_for_each_entry_rcu() {
}
rcu_read_unlock();

Which is really cheap. Did you actually have performance numbers for
this?

A possibly better option is to add a static_branch() that could elide
any and all memory access.