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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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@intel.com>
>>>
>>> Enable cache-aware load balancing only if at least 1 NUMA node has
>>> more than one LLC.
>>>
>>> Suggested-by: Libo Chen <libo.chen@oracle.com>
>>> Suggested-by: Adam Li <adamli@os.amperecomputing.com>
>>> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
>>> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
>>> ---
>>>   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.