Re: [PATCH] nohz: nohz idle balancing per node

[Vincent Guittot]

On Thu, 1 Jul 2021 at 07:53, Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
>
> Currently a single nohz idle CPU is designated to perform balancing on
> behalf of all other nohz idle CPUs in the system. Implement a per node
> nohz balancer to minimize cross-node memory accesses and runqueue lock
> acquisitions.
>
> On a 4 node system, this improves performance by 9.3% on a 'pgbench -N'
> with 32 clients/jobs (which is about where throughput maxes out due to
> IO and contention in postgres).
>
> Performance consistency and drop-off after peak is much improved:
>
> https://user-images.githubusercontent.com/19833002/124070985-0e923a80-da82-11eb-81c8-6788be27dfdb.png
>
> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> ---
>  kernel/sched/core.c |  3 ++
>  kernel/sched/fair.c | 90 +++++++++++++++++++++++++++++++--------------
>  2 files changed, 65 insertions(+), 28 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cf16f8fda9a6..52681f11e98c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9067,6 +9067,9 @@ void __init sched_init(void)
>          */
>         init_idle(current, smp_processor_id());
>
> +       /* Must run before init_sched_fair_class(), which tests housekeeping */
> +       housekeeping_init();

You keep calling it after sched_init() in start_kernel()

> +
>         calc_load_update = jiffies + LOAD_FREQ;
>
>  #ifdef CONFIG_SMP
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fb469b26b00a..832f8673bba1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5722,13 +5722,27 @@ DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);
>
>  #ifdef CONFIG_NO_HZ_COMMON
>
> -static struct {
> +struct nohz {
>         cpumask_var_t idle_cpus_mask;
>         atomic_t nr_cpus;
>         int has_blocked;                /* Idle CPUS has blocked load */
>         unsigned long next_balance;     /* in jiffy units */
>         unsigned long next_blocked;     /* Next update of blocked load in jiffies */
> -} nohz ____cacheline_aligned;
> +} ____cacheline_aligned;
> +
> +static struct nohz **nohz_nodes __ro_after_init;
> +
> +static struct nohz *get_nohz(void)
> +{
> +#ifdef CONFIG_CPU_ISOLATION
> +       /*
> +        * May not have a house keeping CPU per node, do global idle balancing.
> +        */
> +       if (static_branch_unlikely(&housekeeping_overridden))
> +               return nohz_nodes[0];
> +#endif
> +       return nohz_nodes[numa_node_id()];

This assumes that we call get_nohz() from local cpu or at least local
node. Have you checked that this is always the case ?

> +}
>
>  #endif /* CONFIG_NO_HZ_COMMON */
>
> @@ -10217,7 +10231,7 @@ static inline int find_new_ilb(void)
>  {
>         int ilb;
>
> -       for_each_cpu_and(ilb, nohz.idle_cpus_mask,
> +       for_each_cpu_and(ilb, get_nohz()->idle_cpus_mask,
>                               housekeeping_cpumask(HK_FLAG_MISC)) {
>
>                 if (ilb == smp_processor_id())
> @@ -10243,7 +10257,7 @@ static void kick_ilb(unsigned int flags)
>          * not if we only update stats.
>          */
>         if (flags & NOHZ_BALANCE_KICK)
> -               nohz.next_balance = jiffies+1;
> +               get_nohz()->next_balance = jiffies+1;
>
>         ilb_cpu = find_new_ilb();
>
> @@ -10291,14 +10305,14 @@ static void nohz_balancer_kick(struct rq *rq)
>          * None are in tickless mode and hence no need for NOHZ idle load
>          * balancing.
>          */
> -       if (likely(!atomic_read(&nohz.nr_cpus)))
> +       if (likely(!atomic_read(&get_nohz()->nr_cpus)))
>                 return;
>
> -       if (READ_ONCE(nohz.has_blocked) &&
> -           time_after(now, READ_ONCE(nohz.next_blocked)))
> +       if (READ_ONCE(get_nohz()->has_blocked) &&
> +           time_after(now, READ_ONCE(get_nohz()->next_blocked)))
>                 flags = NOHZ_STATS_KICK;
>
> -       if (time_before(now, nohz.next_balance))
> +       if (time_before(now, get_nohz()->next_balance))
>                 goto out;
>
>         if (rq->nr_running >= 2) {
> @@ -10328,7 +10342,7 @@ static void nohz_balancer_kick(struct rq *rq)
>                  * currently idle; in which case, kick the ILB to move tasks
>                  * around.
>                  */
> -               for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
> +               for_each_cpu_and(i, sched_domain_span(sd), get_nohz()->idle_cpus_mask) {
>                         if (sched_asym_prefer(i, cpu)) {
>                                 flags = NOHZ_KICK_MASK;
>                                 goto unlock;
> @@ -10405,8 +10419,8 @@ void nohz_balance_exit_idle(struct rq *rq)
>                 return;
>
>         rq->nohz_tick_stopped = 0;
> -       cpumask_clear_cpu(rq->cpu, nohz.idle_cpus_mask);
> -       atomic_dec(&nohz.nr_cpus);
> +       cpumask_clear_cpu(rq->cpu, get_nohz()->idle_cpus_mask);
> +       atomic_dec(&get_nohz()->nr_cpus);
>
>         set_cpu_sd_state_busy(rq->cpu);
>  }
> @@ -10467,8 +10481,8 @@ void nohz_balance_enter_idle(int cpu)
>
>         rq->nohz_tick_stopped = 1;
>
> -       cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
> -       atomic_inc(&nohz.nr_cpus);
> +       cpumask_set_cpu(cpu, get_nohz()->idle_cpus_mask);
> +       atomic_inc(&get_nohz()->nr_cpus);
>
>         /*
>          * Ensures that if nohz_idle_balance() fails to observe our
> @@ -10484,7 +10498,7 @@ void nohz_balance_enter_idle(int cpu)
>          * Each time a cpu enter idle, we assume that it has blocked load and
>          * enable the periodic update of the load of idle cpus
>          */
> -       WRITE_ONCE(nohz.has_blocked, 1);
> +       WRITE_ONCE(get_nohz()->has_blocked, 1);
>  }
>
>  static bool update_nohz_stats(struct rq *rq)
> @@ -10494,7 +10508,7 @@ static bool update_nohz_stats(struct rq *rq)
>         if (!rq->has_blocked_load)
>                 return false;
>
> -       if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
> +       if (!cpumask_test_cpu(cpu, get_nohz()->idle_cpus_mask))
>                 return false;
>
>         if (!time_after(jiffies, READ_ONCE(rq->last_blocked_load_update_tick)))
> @@ -10532,7 +10546,7 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>          * setting the flag, we are sure to not clear the state and not
>          * check the load of an idle cpu.
>          */
> -       WRITE_ONCE(nohz.has_blocked, 0);
> +       WRITE_ONCE(get_nohz()->has_blocked, 0);
>
>         /*
>          * Ensures that if we miss the CPU, we must see the has_blocked
> @@ -10544,10 +10558,7 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>          * Start with the next CPU after this_cpu so we will end with this_cpu and let a
>          * chance for other idle cpu to pull load.
>          */
> -       for_each_cpu_wrap(balance_cpu,  nohz.idle_cpus_mask, this_cpu+1) {
> -               if (!idle_cpu(balance_cpu))
> -                       continue;
> -
> +       for_each_cpu_wrap(balance_cpu,  get_nohz()->idle_cpus_mask, this_cpu+1) {
>                 /*
>                  * If this CPU gets work to do, stop the load balancing
>                  * work being done for other CPUs. Next load
> @@ -10558,6 +10569,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>                         goto abort;
>                 }
>
> +               if (!idle_cpu(balance_cpu))
> +                       continue;

The move of this test seems to be out of the scope of the patch subject

> +
>                 rq = cpu_rq(balance_cpu);
>
>                 has_blocked_load |= update_nohz_stats(rq);
> @@ -10589,15 +10603,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>          * updated.
>          */
>         if (likely(update_next_balance))
> -               nohz.next_balance = next_balance;
> +               get_nohz()->next_balance = next_balance;
>
> -       WRITE_ONCE(nohz.next_blocked,
> +       WRITE_ONCE(get_nohz()->next_blocked,
>                 now + msecs_to_jiffies(LOAD_AVG_PERIOD));
>
>  abort:
>         /* There is still blocked load, enable periodic update */
>         if (has_blocked_load)
> -               WRITE_ONCE(nohz.has_blocked, 1);
> +               WRITE_ONCE(get_nohz()->has_blocked, 1);
>  }
>
>  /*
> @@ -10655,8 +10669,8 @@ static void nohz_newidle_balance(struct rq *this_rq)
>                 return;
>
>         /* Don't need to update blocked load of idle CPUs*/
> -       if (!READ_ONCE(nohz.has_blocked) ||
> -           time_before(jiffies, READ_ONCE(nohz.next_blocked)))
> +       if (!READ_ONCE(get_nohz()->has_blocked) ||
> +           time_before(jiffies, READ_ONCE(get_nohz()->next_blocked)))
>                 return;
>
>         /*
> @@ -11573,9 +11587,29 @@ __init void init_sched_fair_class(void)
>         open_softirq(SCHED_SOFTIRQ, run_rebalance_domains);
>
>  #ifdef CONFIG_NO_HZ_COMMON
> -       nohz.next_balance = jiffies;
> -       nohz.next_blocked = jiffies;
> -       zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
> +       if (static_branch_unlikely(&housekeeping_overridden)) {
> +               struct nohz *nohz;
> +
> +               nohz_nodes = kcalloc(1, sizeof(struct nohz *), GFP_NOWAIT);
> +               nohz = kmalloc(sizeof(struct nohz), GFP_NOWAIT);
> +               nohz->next_balance = jiffies;
> +               nohz->next_blocked = jiffies;
> +               zalloc_cpumask_var(&nohz->idle_cpus_mask, GFP_NOWAIT);
> +               nohz_nodes[0] = nohz;
> +       } else {
> +               int n;
> +
> +               nohz_nodes = kcalloc(nr_node_ids, sizeof(struct nohz *), GFP_NOWAIT);
> +               for_each_node(n) {
> +                       struct nohz *nohz;
> +
> +                       nohz = kmalloc_node(sizeof(struct nohz), GFP_NOWAIT, n);
> +                       nohz->next_balance = jiffies;
> +                       nohz->next_blocked = jiffies;
> +                       zalloc_cpumask_var_node(&nohz->idle_cpus_mask, GFP_NOWAIT, n);
> +                       nohz_nodes[n] = nohz;
> +               }
> +       }
>  #endif
>  #endif /* SMP */
>
> --
> 2.23.0
>