Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache aware only for multi LLCs

[Shrikanth Hegde]

On 10/15/25 9:55 PM, Chen, Yu C wrote:
> On 10/15/2025 7:04 PM, Peter Zijlstra wrote:
> > On Sat, Oct 11, 2025 at 11:24:41AM -0700, Tim Chen wrote:
> > > From: Chen Yu <yu.c.chen@xxxxxxxxx>
> > >
> > > Enable cache-aware load balancing only if at least 1 NUMA node has
> > > more than one LLC.
> > >
> > > Suggested-by: Libo Chen <libo.chen@xxxxxxxxxx>
> > > Suggested-by: Adam Li <adamli@xxxxxxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > > ---
> > >   kernel/sched/fair.c     | 15 ++++++++++++---
> > >   kernel/sched/sched.h    |  1 +
> > >   kernel/sched/topology.c | 14 ++++++++++++--
> > >   3 files changed, 25 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index cd080468ddc9..3d643449c48c 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -1208,6 +1208,14 @@ static s64 update_se(struct rq *rq, struct 
> > > sched_entity *se)
> > >   __read_mostly unsigned int llc_overload_pct       = 50;
> > >   __read_mostly unsigned int llc_imb_pct            = 20;
> > > +DEFINE_STATIC_KEY_FALSE(sched_cache_allowed);
> > > +
> > > +static inline bool sched_cache_enabled(void)
> > > +{
> > > +    return sched_feat(SCHED_CACHE) &&
> > > +        static_branch_likely(&sched_cache_allowed);
> > > +}
> >
> > Urgh; do we really need _2_ static keys stacked for this? I'm thinking
> > one should be well enough.
>
> SCHED_CACHE allows user space to turn on/off the feature at runtime,
> while sched_cache_allow is a hardware capability. This capability is

isn't it possible use only static_branch_likely(&sched_cache_allowed) at runtime?

Enable that key only if FEAT is set. Disable when unset.
That way you could use only one static branch at runtime.

Also, I am not sure if the FEATURE should be true by default. I know it maybe unused but
IMO it should be true by default only when its proven there are no regression.
One should be aware of their topology to enable it.
>   disabled if there are no multiple LLCs within one node. I’m not sure
> if using one key could support the above two scenarios.
> It is possible to have multiple NUMA nodes. One node may have multiple LLC while other
one may have only 1 LLC. what happens in that case?

I am yet to go through the series(hopefully this week). Maybe its handled already.