Accretion and feedback tied to supermassive black holes play central role in the cosmic evolution of galaxies, groups, and clusters. The self-regulation mechanism, that is how to link feedback and accretion, is matter of intense debate. Using high-resolution 3D hydrodynamic simulations, I discuss how the AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion. In a turbulent atmosphere heated by AGN feedback, cold clouds and filaments condense out of the hot plasma via nonlinear thermal instability up to several 10 kpc radii, and rain through the galaxy toward the black hole. In the inner core, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation, boosting the accretion rate up to two orders of magnitude. Such rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. The AGN heating later stifles the formation of multiphase gas. Lacking the main fuel, AGN feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the black hole and the whole host galaxy, leading to a tight self-regulated feedback loop which preserves the cores of massive galaxies, groups, and clusters in quasi thermal equilibrium throughout cosmic time. I highlight the major imprints of mechanical AGN feedback, as bubbles, shocks, and turbulence, and focus on the multiphase structure of the gaseous atmosphere with a critical eye toward concordance with multiwavelength observations, from the X-ray plasma to the cold gas.