On Tue, Jul 25, 2023 at 11:10:31AM +0100, Valentin Schneider wrote: > I have reasons! I just swept them under the rug and didn't mention them :D > Also looking at the config dependencies again I got it wrong, but > nevertheless that means I get to ramble about it. > > With NO_HZ_IDLE, we get CONTEXT_TRACKING_IDLE, so we get these > transitions: > > ct_idle_enter() > ct_kernel_exit() > ct_state_inc_clear_work() > > ct_idle_exit() > ct_kernel_enter() > ct_work_flush() > > Now, if we just make CONTEXT_TRACKING_WORK depend on CONTEXT_TRACKING_IDLE > rather than CONTEXT_TRACKING_USER, we get to leverage the IPI deferral for > NO_HZ_IDLE kernels - in other words, we get to keep idle CPUs idle longer. > > It's a completely different argument than reducing interference for > NOHZ_FULL userspace applications and I should have at the very least > mentioned it in the cover letter, but it's the exact same backing > mechanism. > > Looking at it again, I'll probably make the CONTEXT_IDLE thing a separate > patch with a proper changelog. Ok should that be a seperate Kconfig? This indeed can bring power improvement but at the cost of more overhead from the sender. A balance to be measured...
