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Message-ID: <d79d6c03-5a6b-46fd-a973-dd941f31cc88@intel.com>
Date: Sat, 6 Dec 2025 10:11:07 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Christian Loehle <christian.loehle@....com>
CC: <qyousef@...alina.io>, <hongyan.xia2@....com>, <luis.machado@....com>,
Vincent Guittot <vincent.guittot@...aro.org>, <mingo@...hat.com>,
<peterz@...radead.org>, <dietmar.eggemann@....com>, <juri.lelli@...hat.com>,
<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
<vschneid@...hat.com>, <pierre.gondois@....com>, <kprateek.nayak@....com>,
<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 5/6 v8] sched/fair: Enable idle core tracking for !SMT
On 12/5/2025 11:52 PM, Christian Loehle wrote:
> 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@...aro.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...
>
update_idle_core() might need to deal with rcu protection in the original
code, maybe something like this would help:
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 9dfabaa314b1..4c9348075abf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2094,17 +2094,22 @@ extern void __update_idle_core(struct rq *rq);
static inline void update_idle_core(struct rq *rq)
{
- if (static_branch_unlikely(&sched_smt_present))
+ if (static_branch_unlikely(&sched_smt_present)) {
__update_idle_core(rq);
- else
+ } else {
+ rcu_read_lock();
set_idle_cores(cpu_of(rq), 1);
+ rcu_read_unlock();
+ }
}
#else /* !CONFIG_SCHED_SMT: */
static inline void update_idle_core(struct rq *rq)
{
+ rcu_read_lock();
set_idle_cores(cpu_of(rq), 1);
+ rcu_read_unlock();
}
#endif /* !CONFIG_SCHED_SMT */
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