On 25/07/23 13:22, Frederic Weisbecker wrote:
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...
Yep agreed, I'll make that an optional config.