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Message-ID: <CAKfTPtCi8OVg+ewVP4G4VKZuoMK00SThexei=M_ZQOYNWdCSEw@mail.gmail.com>
Date: Sat, 6 Dec 2025 11:09:30 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Christian Loehle <christian.loehle@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
pierre.gondois@....com, kprateek.nayak@....com, qyousef@...alina.io,
hongyan.xia2@....com, luis.machado@....com
Subject: Re: [RFC PATCH 5/6 v8] sched/fair: Enable idle core tracking for !SMT
On Fri, 5 Dec 2025 at 16:53, Christian Loehle <christian.loehle@....com> 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
Thanks for the report
> [ 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...
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