Re: [RFC PATCH 4/5] sched: Inhibit cache aware scheduling if the preferred LLC is over aggregated

[Tim Chen]

On Fri, 2025-04-25 at 14:43 +0530, Madadi Vineeth Reddy wrote:
> Hi Tim,
> 
> On 24/04/25 21:21, Tim Chen wrote:
> > On Thu, 2025-04-24 at 22:11 +0800, Chen, Yu C wrote:
> > > 
> > > > It spawns lot of threads and is CPU intensive. So, I think it's not impacted
> > > > due to the below conditions.
> > > > 
> > > > Also, in schbench numbers provided by you, there is a degradation in saturated
> > > > case. Is it due to the overhead in computing the preferred llc which is not
> > > > being used due to below conditions?
> > > 
> > > Yes, the overhead of preferred LLC calculation could be one part, and we 
> > > also suspect that the degradation might be tied to the task migrations. 
> > > We still observed more task migrations than the baseline, even when the 
> > > system was saturated (in theory, after 25% is exceeded, we should 
> > > fallback to the generic task wakeup path). We haven't dug into that yet, 
> > > and we can conduct an investigation in the following days.
> > 
> > In the saturation case it is mostly the tail latency that has regression.
> > The preferred LLC has a tendency to have higher load than the
> > other LLCs. Load balancer will try to move tasks out and wake balance will
> > try to move it back to the preferred LLC. This increases the task migrations
> > and affect tail latency.
> 
> Why would the task be moved back to the preferred LLC in wakeup path for the
> saturated case? The checks shouldn't allow it right?

The task wake ups happens very frequently in schbench and it takes a while for utilization to catch
up. The utilization of the LLC is updated at the load balance time of LLC. 

So once utilization falls below the utilization threshold, there is a window
where the woken tasks will rush into the preferred LLC until the utilization
is updated at the next load balance time. 

Tim


> 
> Thanks,
> Madadi Vineeth Reddy
> 
> > 
> > Tim
>