Re: [PATCH v3 2/8] sched/fair: Maintain the identity of idle-core

[Srikar Dronamraju]

* Vincent Guittot <vincent.guittot@xxxxxxxxxx> [2021-05-21 14:36:15]:

> On Thu, 13 May 2021 at 09:40, Srikar Dronamraju
> <srikar@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > Cc: Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> > Cc: Parth Shah <parth@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > Cc: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
> > Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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
> 

Agree, this is not needed. will fix in the next version.
Thanks for pointing out.

> > +               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 ?
> 

You are right, but this is similar to what we do currently do too. Even
without this patch, we got ahead an unconditionally (We dont even have an
option to see if the selected CPU was from an idle-core.) set the idle-core
to -1. (Please see the hunk I removed below)

I try to improve upon this in the next iteration. But that again we are
seeing some higher utilization probably with that change.

I plan to move to a cpumask based approach in v4.
By which we dont have to search for setting an idle-core but we still know
if any idle-cores are around. However that will have the extra penalty of
atomic operations that you commented to in one of my patches.

But if you have other ideas, I would be willing to try out.

> 
> 
> >                                 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);
> > -

I was referring to this hunk.

> >         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
> >

-- 
Thanks and Regards
Srikar Dronamraju