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Subject[PATCH] nohz: nohz idle balancing per node
Date
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@gmail.com>
---
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();
+
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()];
+}

#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;
+
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
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