Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement)

[Tobias Huschle]

On Sun, Jan 21, 2024 at 01:44:32PM -0500, Michael S. Tsirkin wrote:
> 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.
> 
> Let's try this? Does not look like discussing vhost itself will
> draw attention from scheduler guys but posting a scheduling
> patch probably will? Can you post a patch?
> 

I'll give it a go.

> > - 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.
> > - 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.
> 
> Or this last one.
> 

This one will probably be more complicated as the necessary information
is not really available at the places where I'd like to see it.
Will have to ponder on that a bit to figure out if there might be an
elegant way to approach this.

> 
> > 
> > I would be curious on feedback on those ideas and interested in
> > alternative approaches.
> 
>