Re: [RFC PATCH 5/6 v8] sched/fair: Enable idle core tracking for !SMT

[Christian Loehle <christian.loehle@arm.com>]

On 12/2/25 18:12, Vincent Guittot wrote:
> Enable has_idle_cores at llc level feature for !SMT system for which
> CPU equals core.
> 
> We don't enable has_idle_core feature of select_idle_cpu to be
> conservative and don't parse all CPUs of LLC.
> 
> At now, has_idle_cores can be cleared even if a CPU is idle because of
> SIS_UTIL but it looks reasonnable as the probablity to get an idle CPU is
> low anyway.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c  | 29 +++++++----------------------
>  kernel/sched/sched.h | 42 +++++++++++++++++++++++++++++-------------
>  2 files changed, 36 insertions(+), 35 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 252254168c92..0c0c675f39cf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7501,19 +7501,6 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
>  	return -1;
>  }
>  
> -#ifdef CONFIG_SCHED_SMT
> -DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> -EXPORT_SYMBOL_GPL(sched_smt_present);
> -
> -static inline void set_idle_cores(int cpu, int val)
> -{
> -	struct sched_domain_shared *sds;
> -
> -	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> -	if (sds)
> -		WRITE_ONCE(sds->has_idle_cores, val);
> -}
> -
>  static inline bool test_idle_cores(int cpu)
>  {
>  	struct sched_domain_shared *sds;
> @@ -7525,6 +7512,10 @@ static inline bool test_idle_cores(int cpu)
>  	return false;
>  }
>  
> +#ifdef CONFIG_SCHED_SMT
> +DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> +EXPORT_SYMBOL_GPL(sched_smt_present);
> +
>  /*
>   * Scans the local SMT mask to see if the entire core is idle, and records this
>   * information in sd_llc_shared->has_idle_cores.
> @@ -7612,15 +7603,6 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>  
>  #else /* !CONFIG_SCHED_SMT: */
>  
> -static inline void set_idle_cores(int cpu, int val)
> -{
> -}
> -
> -static inline bool test_idle_cores(int cpu)
> -{
> -	return false;
> -}
> -
>  static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
>  {
>  	return __select_idle_cpu(core, p);
> @@ -7886,6 +7868,9 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  	if ((unsigned)i < nr_cpumask_bits)
>  		return i;
>  
a
> +
>  	/*
>  	 * For cluster machines which have lower sharing cache like L2 or
>  	 * LLC Tag, we tend to find an idle CPU in the target's cluster
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 697bd654298a..b9e228333d5e 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1591,19 +1591,6 @@ do {						\
>  	flags = _raw_spin_rq_lock_irqsave(rq);	\
>  } while (0)
>  
> -#ifdef CONFIG_SCHED_SMT
> -extern void __update_idle_core(struct rq *rq);
> -
> -static inline void update_idle_core(struct rq *rq)
> -{
> -	if (static_branch_unlikely(&sched_smt_present))
> -		__update_idle_core(rq);
> -}
> -
> -#else /* !CONFIG_SCHED_SMT: */
> -static inline void update_idle_core(struct rq *rq) { }
> -#endif /* !CONFIG_SCHED_SMT */
> -
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  
>  static inline struct task_struct *task_of(struct sched_entity *se)
> @@ -2091,6 +2078,35 @@ static __always_inline bool sched_asym_cpucap_active(void)
>  	return static_branch_unlikely(&sched_asym_cpucapacity);
>  }
>  
> +static inline void set_idle_cores(int cpu, int val)
> +{
> +	struct sched_domain_shared *sds;
> +
> +	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +	if (sds)
> +		WRITE_ONCE(sds->has_idle_cores, val);
> +}

FWIW this triggers
[    0.172174] =============================
[    0.172177] WARNING: suspicious RCU usage
[    0.172179] 6.18.0-rc7-cix-build+ #215 Not tainted
[    0.172184] Detected PIPT I-cache on CPU1
[    0.178161] -----------------------------
[    0.178163] kernel/sched/sched.h:2085 suspicious rcu_dereference_check() usage!
[    0.178165] 
               other info that might help us debug this:

[    0.178177] CPU features: SANITY CHECK: Unexpected variation in SYS_ID_AA64MMFR1_EL1. Boot CPU: 0x1001111010312122, CPU1: 0x1001111011312122
[    0.182211] 
               rcu_scheduler_active = 1, debug_locks = 1
[    0.182213] 4 locks held by swapper/0/1:
[    0.182224] CPU features: Unsupported CPU feature variation detected.
[    0.186260]  #0: ffff800082b2bf00
[    0.186277] GICv3: CPU1: found redistributor 0 region 0:0x000000000e090000
[    0.191101]  (cpu_add_remove_lock){+.+.}-{4:4}, at: cpu_up+0x90/0x158
[    0.191115] GICv3: CPU1: using allocated LPI pending table @0x0000000100330000
[    0.195158]  #1: ffff800082b2c0a0 (cpu_hotplug_lock
[    0.195277] CPU1: Booted secondary processor 0x0000000000 [0x410fd801]
[    0.199208] ){++++}-{0:0}, at: _cpu_up+0x58/0x268
[    0.199213]  #2: ffff800082ebddd0 (sparse_irq_lock){+.+.}-{4:4}, at: irq_lock_sparse+0x20/0x2c
[    0.293548]  #3: ffff0001feec1c18 (&rq->__lock){-...}-{2:2}, at: __schedule+0x144/0x1058
[    0.301737] 
               stack backtrace:
[    0.306136] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Tainted: G S                  6.18.0-rc7-cix-build+ #215 PREEMPT 
[    0.306141] Tainted: [S]=CPU_OUT_OF_SPEC
[    0.306144] Call trace:
[    0.306145]  show_stack+0x18/0x24 (C)
[    0.306150]  dump_stack_lvl+0x90/0xd0
[    0.306155]  dump_stack+0x18/0x24
[    0.306159]  lockdep_rcu_suspicious+0x168/0x238
[    0.306164]  set_next_task_idle+0x144/0x148
[    0.306167]  __schedule+0xc50/0x1058
[    0.306171]  schedule+0x48/0x15c
[    0.306173]  schedule_timeout+0x90/0x128
[    0.306177]  wait_for_completion_timeout+0x88/0x13c
[    0.306180]  __cpu_up+0x80/0x1e4
[    0.306186]  bringup_cpu+0x48/0x2ac
[    0.306189]  cpuhp_invoke_callback+0x18c/0x358
[    0.306191]  __cpuhp_invoke_callback_range+0xf4/0x130
[    0.306194]  _cpu_up+0x150/0x268
[    0.306196]  cpu_up+0xcc/0x158
[    0.306199]  bringup_nonboot_cpus+0x84/0xcc
[    0.306203]  smp_init+0x30/0x8c
[    0.306208]  kernel_init_freeable+0x18c/0x504
[    0.306215]  kernel_init+0x20/0x1d8
[    0.306218]  ret_from_fork+0x10/0x20


on my machine...