linux-kernel - Re: [RFC patch v3 08/20] sched: Set up LLC indexing

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Message-ID: <e92d0ccb-733e-4104-bc32-743771bfc2b4@intel.com>
Date: Fri, 4 Jul 2025 17:36:03 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, Tim Chen
	<tim.c.chen@...ux.intel.com>
CC: 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>, Tim Chen <tim.c.chen@...el.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Libo Chen <libo.chen@...cle.com>, Abel Wu
	<wuyun.abel@...edance.com>, Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>, Len Brown <len.brown@...el.com>,
	<linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, "Ingo
 Molnar" <mingo@...hat.com>, K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [RFC patch v3 08/20] sched: Set up LLC indexing

On 7/4/2025 3:44 AM, Shrikanth Hegde wrote:
> 
> 
> On 6/18/25 23:57, Tim Chen wrote:
>> Prepare for indexing arrays that track in each run queue: the number
>> of tasks preferring current LLC and each of the other LLC.
>>
>> The reason to introduce LLC index is because the per LLC-scope data
>> is needed to do cache aware load balancing. However, the native lld_id
>> is usually the first CPU of that LLC domain, which is not continuous,
>> which might waste the space if the per LLC-scope data is stored
>> in an array (in current implementation).
>>
>> In the future, this LLC index could be removed after
>> the native llc_id is used as the key to search into xarray based
>> array.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> ---
>>   include/linux/sched.h   |  3 +++
>>   kernel/sched/fair.c     | 12 ++++++++++++
>>   kernel/sched/sched.h    |  2 ++
>>   kernel/sched/topology.c | 29 +++++++++++++++++++++++++++++
>>   4 files changed, 46 insertions(+)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index d0e4cda2b3cd..7ce95a32e9ff 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -810,6 +810,9 @@ struct kmap_ctrl {
>>   #endif
>>   };
>> +/* XXX need fix to not use magic number */
>> +#define MAX_LLC 64
> 
> This number needs to be much higher. maybe keeping NR_CPUS wont hurt.
> 
It will be replaced by xarray, so above restriction might not be needed
anymore.

>>   }
>> +/*
>> + * continous index.
>> + * TBD: replace by xarray with key llc_id()
>> + */
>> +static inline int llc_idx(int cpu)
>> +{
>> +    if (cpu < 0)
>> +        return -1;
>> +
>> +    return per_cpu(sd_llc_idx, cpu);
>> +}
>> +
>>   void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu 
>> *_pcpu_sched)
>>   {
>>       unsigned long epoch;
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 1c6fd45c7f62..74eb2f3615aa 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -2037,6 +2037,7 @@ static inline struct sched_domain 
>> *lowest_flag_domain(int cpu, int flag)
>>   DECLARE_PER_CPU(struct sched_domain __rcu *, sd_llc);
>>   DECLARE_PER_CPU(int, sd_llc_size);
>>   DECLARE_PER_CPU(int, sd_llc_id);
>> +DECLARE_PER_CPU(int, sd_llc_idx);
>>   DECLARE_PER_CPU(int, sd_share_id);
>>   DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>>   DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
>> @@ -2045,6 +2046,7 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, 
>> sd_asym_cpucapacity);
>>   extern struct static_key_false sched_asym_cpucapacity;
>>   extern struct static_key_false sched_cluster_active;
>> +extern int max_llcs;
>>   static __always_inline bool sched_asym_cpucap_active(void)
>>   {
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index f1ebc60d967f..b7bb13045dd8 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -672,6 +672,7 @@ static void destroy_sched_domains(struct 
>> sched_domain *sd)
>>   DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc);
>>   DEFINE_PER_CPU(int, sd_llc_size);
>>   DEFINE_PER_CPU(int, sd_llc_id);
>> +DEFINE_PER_CPU(int, sd_llc_idx);
>>   DEFINE_PER_CPU(int, sd_share_id);
>>   DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
>>   DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
>> @@ -681,6 +682,25 @@ DEFINE_PER_CPU(struct sched_domain __rcu *, 
>> sd_asym_cpucapacity);
>>   DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
>>   DEFINE_STATIC_KEY_FALSE(sched_cluster_active);
>> +int max_llcs = -1;
>> +
>> +static void update_llc_idx(int cpu)
>> +{
>> +#ifdef CONFIG_SCHED_CACHE
>> +    int idx = -1, llc_id = -1;
>> +
>> +    llc_id = per_cpu(sd_llc_id, cpu);
>> +    idx = per_cpu(sd_llc_idx, llc_id);
>> +
>> +    if (idx < 0) {
>> +        idx = max_llcs++;
>> +        BUG_ON(idx > MAX_LLC); 
> 
> maybe a warning instead here?
> 

Ditto.


thanks,
Chenyu