On Mon, Apr 20, 2026 at 4:45 PM Peter Zijlstra <[email protected]> wrote: > > On Mon, Apr 20, 2026 at 02:28:04PM +0200, Albert Esteve wrote: > > From: Alessandro Carminati <[email protected]> > > > > 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 <[email protected]> > > Signed-off-by: Albert Esteve <[email protected]> > > --- > > 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?
No, I do not have performance numbers. I kept the counter and the separate patch for consistency with the previous version of the series. But you have a good point, the skipped part is really cheap. > > A possibly better option is to add a static_branch() that could elide > any and all memory access. > Previous version had static_branch and I removed it because I understood from the discussion that the gains would not be significant as performance gains are irrelevant in warn slowpath. But I think it would make sense for a disabled feature. I will rework this for the next version, remove the counter and use static_branch as suggested.
