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Message-ID: <1f7e5b4f-a3c3-4505-ae83-bf7dea918571@linux.ibm.com>
Date: Fri, 4 Jul 2025 01:14:09 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>, Chen Yu <yu.c.chen@...el.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 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.
> +
> struct task_struct {
> #ifdef CONFIG_THREAD_INFO_IN_TASK
> /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 10ea408d0e40..5549710d95cf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1183,6 +1183,18 @@ static int llc_id(int cpu)
> return per_cpu(sd_llc_id, cpu);
> }
>
> +/*
> + * 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?
> + per_cpu(sd_llc_idx, llc_id) = idx;
> + }
> + per_cpu(sd_llc_idx, cpu) = idx;
> +#endif
> +}
> +
> static void update_top_cache_domain(int cpu)
> {
> struct sched_domain_shared *sds = NULL;
> @@ -699,6 +719,7 @@ static void update_top_cache_domain(int cpu)
> per_cpu(sd_llc_size, cpu) = size;
> per_cpu(sd_llc_id, cpu) = id;
> rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
> + update_llc_idx(cpu);
>
> sd = lowest_flag_domain(cpu, SD_CLUSTER);
> if (sd)
> @@ -2394,6 +2415,14 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> bool has_asym = false;
> bool has_cluster = false;
>
> +#ifdef CONFIG_SCHED_CACHE
> + if (max_llcs < 0) {
> + for_each_possible_cpu(i)
> + per_cpu(sd_llc_idx, i) = -1;
> + max_llcs = 0;
> + }
> +#endif
> +
> if (WARN_ON(cpumask_empty(cpu_map)))
> goto error;
>
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