Re: [PATCH v3 00/21] Cache Aware Scheduling

[Chen, Yu C]

Hi Peter, Qais,

On 2/19/2026 10:41 PM, Peter Zijlstra wrote:
> On Thu, Feb 19, 2026 at 02:08:28PM +0000, Qais Yousef wrote:
> > On 02/10/26 14:18, Tim Chen wrote:

[ ... ]

> > I admit yet to look fully at the series. But I must ask, why are you deferring
> > to load balance and not looking at wake up path? LB should be for corrections.
> > When wake up path is doing wrong decision all the time, LB (which is super slow
> > to react) is too late to start grouping tasks? What am I missing?
>
> There used to be wakeup steering, but I'm not sure that still exists in
> this version (still need to read beyond the first few patches). It isn't
> hard to add.

Please let me explain a little more about why we did this in the
load balance path. Yes, the original version implemented cache-aware
scheduling only in the wakeup path. According to our testing, this appeared
to cause some task bouncing issues across LLCs. This was due to conflicts
with the legacy load balancer, which tries to spread tasks to different 
LLCs.
So as Peter said, the load balancer should be taken care of anyway. Later,
we kept only the cache aware logic in the load balancer, and the test 
results
became much more stable, so we kept it as is. The wakeup path more or less
aggregates the wakees(threads within the same process) within the LLC in 
the
wakeup fast path, so we have not changed it for now.

Let me copy the changelog from the previous patch version:

"
In previous versions, aggregation of tasks were done in the
wake up path, without making load balancing paths aware of
LLC (Last-Level-Cache) preference. This led to the following
problems:

1) Aggregation of tasks during wake up led to load imbalance
   between LLCs
2) Load balancing tried to even out the load between LLCs
3) Wake up tasks aggregation happened at a faster rate and
   load balancing moved tasks in opposite directions, leading
   to continuous and excessive task migrations and regressions
   in benchmarks like schbench.

In this version, load balancing is made cache-aware. The main
idea of cache-aware load balancing consists of two parts:

1) Identify tasks that prefer to run on their hottest LLC and
   move them there.
2) Prevent generic load balancing from moving a task out of
   its hottest LLC.
"

thanks,
Chenyu