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

[Peter Zijlstra <peterz@infradead.org>]

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.