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

From: Chen, Yu C

Date: Fri Dec 05 2025 - 21:11:28 EST

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@xxxxxxxxxx>
---
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 */