Whatever stops the threat, and ideally, whatever delays or prevents the threat from occurring.

Knives are good at causing harm and they are almost ideal for assassinations, however, they are horrible for "home defence" or "self defence". If you are using a knife to truly defend yourself, it is a last ditch effort against a someone you think is a threat to your life. Rushing them with a knife can work, but it is very, very dangerous and doesn't end the threat. So if you want to rush and tackling or stab an active shooter in a desperate attempt to stop the threat, be a hero. Many get hurt or killed doing that, but many do succeed in the attempt.

If you want to engage with a knife when you feel your life isn't risked by doing so, that is called "assassination" or "fighting" and it is not defense.