Log since RFC:
1. Throttle only when wake-affine failed. (thanks to PeterZ)
2. Do throttle inside wake_affine(). (thanks to PeterZ)
3. Other small fix.
Recently testing show that wake-affine stuff cause regression on pgbench, the
hiding rat was finally catched out.
wake-affine stuff is always trying to pull wakee close to waker, by theory,
this will benefit us if waker's cpu cached hot data for wakee, or the extreme
ping-pong case.
However, the whole stuff is somewhat blindly, load balance is the only factor
to be guaranteed, and since the stuff itself is time-consuming, some workload
suffered, pgbench is just the one who has been found.
Thus, throttle the wake-affine stuff for such workload is necessary.
This patch introduced a new knob 'sysctl_sched_wake_affine_interval' with the
default value 1ms (default minimum balance interval), which means wake-affine
will keep silent for 1ms after it returned false.
By turning the new knob, those workload who suffered will have the chance to
stop the regression.
Test:
Test with 12 cpu X86 server and tip 3.9.0-rc2.
default
base 1ms interval 10ms interval 100ms interval
| db_size | clients | tps | | tps | | tps | | tps |
+---------+---------+-------+- +-------+ +-------+ +-------+
| 21 MB | 1 | 10572 | | 10804 | | 10802 | | 10801 |
| 21 MB | 2 | 21275 | | 21533 | | 21400 | | 21498 |
| 21 MB | 4 | 41866 | | 42158 | | 42410 | | 42306 |
| 21 MB | 8 | 53931 | | 55796 | | 58608 | +8.67% | 59916 |
+11.10%
| 21 MB | 12 | 50956 | | 52266 | | 54586 | +7.12% | 55982 |
+9.86%
| 21 MB | 16 | 49911 | | 52862 | +5.91% | 55668 | +11.53% | 57255 |
+14.71%
| 21 MB | 24 | 46046 | | 48820 | +6.02% | 54269 | +17.86% | 58113 |
+26.21%
| 21 MB | 32 | 43405 | | 46635 | +7.44% | 53690 | +23.70% | 57729 |
+33.00%
| 7483 MB | 1 | 7734 | | 8013 | | 8046 | | 7879 |
| 7483 MB | 2 | 19375 | | 19459 | | 19448 | | 19421 |
| 7483 MB | 4 | 37408 | | 37780 | | 37937 | | 37819 |
| 7483 MB | 8 | 49033 | | 50389 | | 51636 | +5.31% | 52294 |
+6.65%
| 7483 MB | 12 | 45525 | | 47794 | +4.98% | 49828 | +9.45% | 50571 |
+11.08%
| 7483 MB | 16 | 45731 | | 47921 | +4.79% | 50203 | +9.78% | 52033 |
+13.78%
| 7483 MB | 24 | 41533 | | 44301 | +6.67% | 49697 | +19.66% | 53833 |
+29.62%
| 7483 MB | 32 | 36370 | | 38301 | +5.31% | 48146 | +32.38% | 52795 |
+45.16%
| 15 GB | 1 | 7576 | | 7926 | | 7722 | | 7969 |
| 15 GB | 2 | 19157 | | 19284 | | 19294 | | 19304 |
| 15 GB | 4 | 37285 | | 37539 | | 37281 | | 37508 |
| 15 GB | 8 | 48718 | | 49176 | | 50836 | +4.35% | 51239 |
+5.17%
| 15 GB | 12 | 45167 | | 47180 | +4.45% | 49206 | +8.94% | 50126 |
+10.98%
| 15 GB | 16 | 45270 | | 47293 | +4.47% | 49638 | +9.65% | 51748 |
+14.31%
| 15 GB | 24 | 40984 | | 43366 | +5.81% | 49356 | +20.43% | 53157 |
+29.70%
| 15 GB | 32 | 35918 | | 37632 | +4.77% | 47923 | +33.42% | 52241 |
+45.45%
Suggested-by: Peter Zijlstra <a.p.zijls...@chello.nl>
Signed-off-by: Michael Wang <wang...@linux.vnet.ibm.com>
---
include/linux/sched.h | 5 +++++
kernel/sched/fair.c | 31 +++++++++++++++++++++++++++++++
kernel/sysctl.c | 10 ++++++++++
3 files changed, 46 insertions(+), 0 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d35d2b6..e9efd3a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1197,6 +1197,10 @@ enum perf_event_task_context {
perf_nr_task_contexts,
};
+#ifdef CONFIG_SMP
+extern unsigned int sysctl_sched_wake_affine_interval;
+#endif
+
struct task_struct {
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
void *stack;
@@ -1207,6 +1211,7 @@ struct task_struct {
#ifdef CONFIG_SMP
struct llist_node wake_entry;
int on_cpu;
+ unsigned long next_wake_affine;
#endif
int on_rq;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a33e59..68eedd7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3087,6 +3087,22 @@ static inline unsigned long effective_load(struct
task_group *tg, int cpu,
#endif
+/*
+ * Default is 1ms, to prevent the wake_affine() stuff working too frequently.
+ */
+unsigned int sysctl_sched_wake_affine_interval = 1U;
+
+static inline int wake_affine_throttled(struct task_struct *p)
+{
+ return time_before(jiffies, p->next_wake_affine);
+}
+
+static inline void wake_affine_throttle(struct task_struct *p)
+{
+ p->next_wake_affine = jiffies +
+ msecs_to_jiffies(sysctl_sched_wake_affine_interval);
+}
+
static int wake_affine(struct sched_domain *sd, struct task_struct *p, int
sync)
{
s64 this_load, load;
@@ -3096,6 +3112,9 @@ static int wake_affine(struct sched_domain *sd, struct
task_struct *p, int sync)
unsigned long weight;
int balanced;
+ if (wake_affine_throttled(p))
+ return 0;
+
idx = sd->wake_idx;
this_cpu = smp_processor_id();
prev_cpu = task_cpu(p);
@@ -3167,6 +3186,18 @@ static int wake_affine(struct sched_domain *sd, struct
task_struct *p, int sync)
return 1;
}
+
+ /*
+ * wake_affine() stuff try to pull wakee to the cpu
+ * around waker, this will benefit us if the data
+ * cached on waker cpu is hot for wakee, or the extreme
+ * ping-pong case.
+ *
+ * However, do such blindly work too frequently will
+ * cause regression to some workload, thus, each time
+ * when wake_affine() failed, throttle it for a while.
+ */
+ wake_affine_throttle(p);
return 0;
}
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index afc1dc6..6ebfc18 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -437,6 +437,16 @@ static struct ctl_table kern_table[] = {
.extra1 = &one,
},
#endif
+#ifdef CONFIG_SMP
+ {
+ .procname = "sched_wake_affine_interval",
+ .data = &sysctl_sched_wake_affine_interval,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ },
+#endif
#ifdef CONFIG_PROVE_LOCKING
{
.procname = "prove_locking",
--
1.7.4.1
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