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Subject[PATCH 03/10] sched/fair: Update idle-core more often
Date
Currently when the scheduler does a load balance and pulls a task or
when a CPU picks up a task during wakeup without having to call
select_idle_cpu(), it never checks if the target CPU is part of the
idle-core. This makes idle-core less accurate.

Given that the identity of idle-core for LLC is maintained, its easy to
update the idle-core as soon as the CPU picks up a task.

This change will update the idle-core whenever a CPU from the idle-core
picks up a task. However if there are multiple idle-cores in the LLC,
and waking CPU happens to be part of the designated idle-core, idle-core
is set to -1 (i.e there are no idle-cores).

To reduce this case, whenever a CPU updates idle-core, it will look for
other cores in the LLC for an idle-core, if the core to which it belongs
to is not idle.

Cc: LKML <linux-kernel@vger.kernel.org>
Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
Cc: Parth Shah <parth@linux.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
kernel/sched/fair.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
kernel/sched/idle.c | 6 ++++++
kernel/sched/sched.h | 2 ++
3 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03083eacdaf0..09c33cca0349 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6037,6 +6037,39 @@ static inline int get_idle_core(int cpu, int def)
return def;
}

+static void set_next_idle_core(struct sched_domain *sd, int target)
+{
+ struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+ int core, cpu;
+
+ cpumask_andnot(cpus, sched_domain_span(sd), cpu_smt_mask(target));
+ for_each_cpu_wrap(core, cpus, target) {
+ bool idle = true;
+
+ for_each_cpu(cpu, cpu_smt_mask(core)) {
+ if (!available_idle_cpu(cpu)) {
+ idle = false;
+ break;
+ }
+ }
+
+ if (idle) {
+ set_idle_core(core, per_cpu(smt_id, core));
+ return;
+ }
+
+ cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
+ }
+}
+
+void set_core_busy(int core)
+{
+ rcu_read_lock();
+ if (get_idle_core(core, -1) == per_cpu(smt_id, core))
+ set_idle_core(core, -1);
+ rcu_read_unlock();
+}
+
/*
* Scans the local SMT mask to see if the entire core is idle, and records this
* information in sd_llc_shared->idle_core.
@@ -6046,11 +6079,13 @@ static inline int get_idle_core(int cpu, int def)
*/
void __update_idle_core(struct rq *rq)
{
+ struct sched_domain *sd;
int core = cpu_of(rq);
int cpu;

rcu_read_lock();
- if (get_idle_core(core, 0) != -1)
+ sd = rcu_dereference(per_cpu(sd_llc, core));
+ if (!sd || get_idle_core(core, 0) != -1)
goto unlock;

for_each_cpu(cpu, cpu_smt_mask(core)) {
@@ -6058,10 +6093,15 @@ void __update_idle_core(struct rq *rq)
continue;

if (!available_idle_cpu(cpu))
- goto unlock;
+ goto try_next;
}

set_idle_core(core, per_cpu(smt_id, core));
+ goto unlock;
+
+try_next:
+ set_next_idle_core(sd, core);
+
unlock:
rcu_read_unlock();
}
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 7199e6f23789..cc828f3efe71 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -425,6 +425,12 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl

static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
{
+#ifdef CONFIG_SCHED_SMT
+ int cpu = rq->cpu;
+
+ if (static_branch_likely(&sched_smt_present))
+ set_core_busy(cpu);
+#endif
}

static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 46d40a281724..5c0bd4b0e73a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1102,6 +1102,7 @@ static inline bool is_migration_disabled(struct task_struct *p)

#ifdef CONFIG_SCHED_SMT
extern void __update_idle_core(struct rq *rq);
+extern void set_core_busy(int cpu);

static inline void update_idle_core(struct rq *rq)
{
@@ -1111,6 +1112,7 @@ static inline void update_idle_core(struct rq *rq)

#else
static inline void update_idle_core(struct rq *rq) { }
+static inline void set_core_busy(int cpu) { }
#endif

DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
--
2.18.2
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