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Message-ID: <CAKfTPtCHb=tjZT60s_Gyp-mfO-rS57XUfn2hXEftJgtMBubA-g@mail.gmail.com>
Date: Fri, 21 May 2021 14:36:15 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Mel Gorman <mgorman@...hsingularity.net>,
Rik van Riel <riel@...riel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <valentin.schneider@....com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
Parth Shah <parth@...ux.ibm.com>,
Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH v3 2/8] sched/fair: Maintain the identity of idle-core
On Thu, 13 May 2021 at 09:40, Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> Scheduler maintains a per LLC info which tells if there is any idle core
> in that LLC. However this information doesn't provide which core is idle.
>
> So when iterating for idle-cores, if select_idle_core() finds an
> idle-core, then it doesn't try to reset this information.
>
> So if there was only one idle core in the LLC and select_idle_core()
> selected the idle-core, the LLC will maintain that it still has a
> idle-core.
>
> On the converse, if a task is pinned, and has a restricted
> cpus_allowed_list and LLC has multiple idle-cores, but select_idle_core
> cannot find a idle-core, LLC will no more maintain that it has an
> idle-core.
>
> As a first step to solve this problem, LLC will maintain the identity of
> the idle core instead of just the information that LLC has an idle core
>
> Along with maintaining, this change will solve both the problems listed
> above. However there are other problems that exist with the current
> infrastructure and those will continue to exist with this change and
> would be handled in subsequent patches.
>
> Cc: LKML <linux-kernel@...r.kernel.org>
> Cc: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
> Cc: Parth Shah <parth@...ux.ibm.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Valentin Schneider <valentin.schneider@....com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Aubrey Li <aubrey.li@...ux.intel.com>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> ---
> Changelog v2->v3:
> - Rebase to tip/sched/core
> (Valentin)
>
> include/linux/sched/topology.h | 2 +-
> kernel/sched/fair.c | 52 ++++++++++++++++++----------------
> kernel/sched/sched.h | 3 ++
> kernel/sched/topology.c | 7 +++++
> 4 files changed, 39 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 8f0f778b7c91..285165a35f21 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -73,7 +73,7 @@ struct sched_group;
> struct sched_domain_shared {
> atomic_t ref;
> atomic_t nr_busy_cpus;
> - int has_idle_cores;
> + int idle_core;
> };
>
> struct sched_domain {
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7920f2a4d257..c42b2b3cd08f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1578,11 +1578,11 @@ numa_type numa_classify(unsigned int imbalance_pct,
>
> #ifdef CONFIG_SCHED_SMT
> /* Forward declarations of select_idle_sibling helpers */
> -static inline bool test_idle_cores(int cpu, bool def);
> +static inline int get_idle_core(int cpu, int def);
> static inline int numa_idle_core(int idle_core, int cpu)
> {
> if (!static_branch_likely(&sched_smt_present) ||
> - idle_core >= 0 || !test_idle_cores(cpu, false))
> + idle_core >= 0 || get_idle_core(cpu, -1) < 0)
> return idle_core;
>
> /*
> @@ -6039,29 +6039,31 @@ static inline int __select_idle_cpu(int cpu)
> DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> EXPORT_SYMBOL_GPL(sched_smt_present);
>
> -static inline void set_idle_cores(int cpu, int val)
> +static inline void set_idle_core(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);
> + WRITE_ONCE(sds->idle_core, val);
> }
>
> -static inline bool test_idle_cores(int cpu, bool def)
> +static inline int get_idle_core(int cpu, int def)
> {
> struct sched_domain_shared *sds;
>
> - sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> - if (sds)
> - return READ_ONCE(sds->has_idle_cores);
> + if (static_branch_likely(&sched_smt_present)) {
Would be good to explain why it is needed to add back the statis branch
> + sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> + if (sds)
> + return READ_ONCE(sds->idle_core);
> + }
>
> return def;
> }
>
> /*
> * Scans the local SMT mask to see if the entire core is idle, and records this
> - * information in sd_llc_shared->has_idle_cores.
> + * information in sd_llc_shared->idle_core.
> *
> * Since SMT siblings share all cache levels, inspecting this limited remote
> * state should be fairly cheap.
> @@ -6072,7 +6074,7 @@ void __update_idle_core(struct rq *rq)
> int cpu;
>
> rcu_read_lock();
> - if (test_idle_cores(core, true))
> + if (get_idle_core(core, 0) >= 0)
> goto unlock;
>
> for_each_cpu(cpu, cpu_smt_mask(core)) {
> @@ -6083,7 +6085,7 @@ void __update_idle_core(struct rq *rq)
> goto unlock;
> }
>
> - set_idle_cores(core, 1);
> + set_idle_core(core, per_cpu(smt_id, core));
> unlock:
> rcu_read_unlock();
> }
> @@ -6091,7 +6093,7 @@ void __update_idle_core(struct rq *rq)
> /*
> * Scan the entire LLC domain for idle cores; this dynamically switches off if
> * there are no idle cores left in the system; tracked through
> - * sd_llc->shared->has_idle_cores and enabled through update_idle_core() above.
> + * sd_llc->shared->idle_core and enabled through update_idle_core() above.
> */
> static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
> {
> @@ -6144,11 +6146,11 @@ 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 void set_idle_core(int cpu, int val)
> {
> }
>
> -static inline bool test_idle_cores(int cpu, bool def)
> +static inline bool get_idle_core(int cpu, int def)
> {
> return def;
> }
> @@ -6170,10 +6172,11 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
> * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> * average idle time for this rq (as found in rq->avg_idle).
> */
> -static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int target)
> +static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int idle_core, int target)
> {
> struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> int i, cpu, idle_cpu = -1, nr = INT_MAX;
> + bool has_idle_core = (idle_core != -1);
> int this = smp_processor_id();
> struct sched_domain *this_sd;
> u64 time;
> @@ -6206,8 +6209,13 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> for_each_cpu_wrap(cpu, cpus, target) {
> if (has_idle_core) {
> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> - if ((unsigned int)i < nr_cpumask_bits)
> + if ((unsigned int)i < nr_cpumask_bits) {
> +#ifdef CONFIG_SCHED_SMT
> + if ((per_cpu(smt_id, i)) == idle_core)
> + set_idle_core(i, -1);
> +#endif
CPUA-core0 enters idle
All other CPUs of core0 are already idle
set idle_core = core0
CPUB-core1 enters idle
All other CPUs of core1 are already idle so core1 becomes idle
A task wakes up and select_idle_core returns CPUA-core0
then idle_core=-1
At next wake up, we skip select_idlecore whereas core1 is idle
Do I miss something ?
> return i;
> + }
>
> } else {
> if (!--nr)
> @@ -6218,9 +6226,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> }
> }
>
> - if (has_idle_core)
> - set_idle_cores(this, false);
> -
> if (sched_feat(SIS_PROP) && !has_idle_core) {
> time = cpu_clock(this) - time;
> update_avg(&this_sd->avg_scan_cost, time);
> @@ -6276,10 +6281,9 @@ static inline bool asym_fits_capacity(int task_util, int cpu)
> */
> static int select_idle_sibling(struct task_struct *p, int prev, int target)
> {
> - bool has_idle_core = false;
> + int i, recent_used_cpu, idle_core = -1;
> struct sched_domain *sd;
> unsigned long task_util;
> - int i, recent_used_cpu;
>
> /*
> * On asymmetric system, update task utilization because we will check
> @@ -6357,16 +6361,16 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> return target;
>
> if (sched_smt_active()) {
> - has_idle_core = test_idle_cores(target, false);
> + idle_core = get_idle_core(target, -1);
>
> - if (!has_idle_core && cpus_share_cache(prev, target)) {
> + if (idle_core < 0 && cpus_share_cache(prev, target)) {
> i = select_idle_smt(p, sd, prev);
> if ((unsigned int)i < nr_cpumask_bits)
> return i;
> }
> }
>
> - i = select_idle_cpu(p, sd, has_idle_core, target);
> + i = select_idle_cpu(p, sd, idle_core, target);
> if ((unsigned)i < nr_cpumask_bits)
> return i;
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a189bec13729..22fbb50b036e 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1491,6 +1491,9 @@ 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);
> +#ifdef CONFIG_SCHED_SMT
> +DECLARE_PER_CPU(int, smt_id);
> +#endif
> DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 55a0a243e871..232fb261dfc2 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -644,6 +644,9 @@ 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);
> +#ifdef CONFIG_SCHED_SMT
> +DEFINE_PER_CPU(int, smt_id);
> +#endif
> DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> @@ -667,6 +670,9 @@ static void update_top_cache_domain(int cpu)
> rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
> per_cpu(sd_llc_size, cpu) = size;
> per_cpu(sd_llc_id, cpu) = id;
> +#ifdef CONFIG_SCHED_SMT
> + per_cpu(smt_id, cpu) = cpumask_first(cpu_smt_mask(cpu));
> +#endif
> rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
>
> sd = lowest_flag_domain(cpu, SD_NUMA);
> @@ -1497,6 +1503,7 @@ sd_init(struct sched_domain_topology_level *tl,
> sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
> atomic_inc(&sd->shared->ref);
> atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
> + sd->shared->idle_core = -1;
> }
>
> sd->private = sdd;
> --
> 2.18.2
>
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