Re: [RFC][PATCH 0/5] preempt_count rework

Mike Galbraith
2013-08-15T09:03:01Z

On Wed, 2013-08-14 at 08:43 -0700, H. Peter Anvin wrote:
> On 08/14/2013 08:39 AM, Mike Galbraith wrote:
> >
> > ..so could the rq = cpu_rq(cpu) sequence be improved cycle expenditure
> > wise by squirreling rq pointer away in a percpu this_rq, and replacing
> > cpu_rq(cpu) above with a __this_cpu_read(this_rq) version of this_rq()?
> >
>
> Yes.

Oh darn, that worked out about as you'd expect. Cycles are so far down
in the frog hair as to be invisible, so not be worth the space cost.

pinned sched_yield proggy, switches/sec, 3 boots/5 runs each:
avg
pre: 1650522 1580422 1604430 1611697 1612928 1611999.8
1682789 1609103 1603866 1559040 1607424 1612444.4
1608265 1607513 1606730 1607079 1635914 1613100.2
1612514.8 avg avg 1.000

post: 1649396 1595364 1621720 1643665 1641829 1630394.8
1571322 1591638 1575406 1629960 1592129 1592091.0
1641807 1622591 1620581 1651145 1663025 1639829.8
1620771.8 avg avg 1.005

---
kernel/sched/core.c | 8 ++++----
kernel/sched/sched.h | 12 +++++++++---
2 files changed, 13 insertions(+), 7 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -111,6 +111,7 @@ void start_bandwidth_timer(struct hrtime

DEFINE_MUTEX(sched_domains_mutex);
DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
+DEFINE_PER_CPU_SHARED_ALIGNED(struct rq *, runqueue);

static void update_rq_clock_task(struct rq *rq, s64 delta);

@@ -2390,7 +2391,7 @@ static void __sched __schedule(void)
need_resched:
preempt_disable();
cpu = smp_processor_id();
- rq = cpu_rq(cpu);
+ rq = this_rq();
rcu_note_context_switch(cpu);
prev = rq->curr;

@@ -2447,8 +2448,7 @@ static void __sched __schedule(void)
* this task called schedule() in the past. prev == current
* is still correct, but it can be moved to another cpu/rq.
*/
- cpu = smp_processor_id();
- rq = cpu_rq(cpu);
+ rq = this_rq();
} else
raw_spin_unlock_irq(&rq->lock);

@@ -6470,7 +6470,7 @@ void __init sched_init(void)
for_each_possible_cpu(i) {
struct rq *rq;

- rq = cpu_rq(i);
+ rq = per_cpu(runqueue, i) = &per_cpu(runqueues, i);
raw_spin_lock_init(&rq->lock);
rq->nr_running = 0;
rq->calc_load_active = 0;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -537,11 +537,17 @@ static inline int cpu_of(struct rq *rq)

DECLARE_PER_CPU(struct rq, runqueues);

-#define cpu_rq(cpu) (&per_cpu(runqueues, (cpu)))
-#define this_rq() (&__get_cpu_var(runqueues))
+/*
+ * Runqueue pointer for use by macros to avoid costly code generated
+ * by taking the address of percpu variables.
+ */
+DECLARE_PER_CPU(struct rq *, runqueue);
+
+#define cpu_rq(cpu) (per_cpu(runqueue, (cpu)))
+#define this_rq() (__this_cpu_read(runqueue))
#define task_rq(p) cpu_rq(task_cpu(p))
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)
-#define raw_rq() (&__raw_get_cpu_var(runqueues))
+#define raw_rq() (__raw_get_cpu_var(runqueue))

static inline u64 rq_clock(struct rq *rq)
{