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

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Message-ID: <6007f50a-7abd-46c8-8934-f1b09df57479@linux.ibm.com>
Date: Wed, 15 Oct 2025 22:06:39 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>, Peter Zijlstra
 <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
        Hillf Danton <hdanton@...a.com>, Jianyong Wu <jianyong.wu@...look.com>,
        Yangyu Chen <cyy@...self.name>, Tingyin Duan <tingyin.duan@...il.com>,
        Vern Hao <vernhao@...cent.com>, Len Brown <len.brown@...el.com>,
        Aubrey Li <aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>,
        Chen Yu <yu.chen.surf@...il.com>,
        Adam Li <adamli@...amperecomputing.com>,
        Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache
 aware only for multi LLCs



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