In recent years, China has become the largest trade partner of North Korea, which suggests how important China is to North Korea. From Asia Times:
It will take decades before the relevant papers will become de-classified, but it seems likely that at some point in 2002 or 2003 the Chinese leadership had an important discussion of the North Korean situation. We can even surmise which decisions were made on that discussion or soon afterwards, even though the exact wording will be known, perhaps, only to our grandchildren. It seems that around 2002 Chinese diplomats and politicians concluded that the collapse of the North Korean state would not serve China’s interests and thus should be prevented or controlled.
Indeed, from around 2002 Chinese investment in the North, as well as Chinese trade with that small and secluded state, began to grow with remarkable speed. In recent years, China has become the largest trade partner of North Korea, controlling about a half of its entire trade volume.
See also BBC’s report: US, N Korea envoys meet in China