On Mon, Jan 08, 2024 at 02:13:25PM +0100, Tobias Huschle wrote: > On Thu, Dec 14, 2023 at 02:14:59AM -0500, Michael S. Tsirkin wrote: > > > > Peter, would appreciate feedback on this. When is cond_resched() > > insufficient to give up the CPU? Should Documentation/kernel-hacking/hacking.rst > > be updated to require schedule() instead? > > > > Happy new year everybody! > > I'd like to bring this thread back to life. To reiterate: > > - The introduction of the EEVDF scheduler revealed a performance > regression in a uperf testcase of ~50%. > - Tracing the scheduler showed that it takes decisions which are > in line with its design. > - The traces showed as well, that a vhost instance might run > excessively long on its CPU in some circumstance. Those cause > the performance regression as they cause delay times of 100+ms > for a kworker which drives the actual network processing. > - Before EEVDF, the vhost would always be scheduled off its CPU > in favor of the kworker, as the kworker was being woken up and > the former scheduler was giving more priority to the woken up > task. With EEVDF, the kworker, as a long running process, is > able to accumulate negative lag, which causes EEVDF to not > prefer it on its wake up, leaving the vhost running. > - If the kworker is not scheduled when being woken up, the vhost > continues looping until it is migrated off the CPU. > - The vhost offers to be scheduled off the CPU by calling > cond_resched(), but, the the need_resched flag is not set, > therefore cond_resched() does nothing. > > To solve this, I see the following options > (might not be a complete nor a correct list) > - Along with the wakeup of the kworker, need_resched needs to > be set, such that cond_resched() triggers a reschedule. > - The vhost calls schedule() instead of cond_resched() to give up > the CPU. This would of course be a significantly stricter > approach and might limit the performance of vhost in other cases. And on these two, I asked: Would appreciate feedback on this. When is cond_resched() insufficient to give up the CPU? Should Documentation/kernel-hacking/hacking.rst be updated to require schedule() instead? > - Preventing the kworker from accumulating negative lag as it is > mostly not runnable and if it runs, it only runs for a very short > time frame. This might clash with the overall concept of EEVDF. > - On cond_resched(), verify if the consumed runtime of the caller > is outweighing the negative lag of another process (e.g. the > kworker) and schedule the other process. Introduces overhead > to cond_resched. > > I would be curious on feedback on those ideas and interested in > alternative approaches.
