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Subject[patch] remove wake-balancing

* Chen, Kenneth W <kenneth.w.chen@intel.com> wrote:

> > If we can get performance to within a couple of tenths of a percent
> > of the zero balancing case, then that would be preferable I think.
>
> I won't try to compromise between the two. If you do so, we would end
> up with two half baked raw turkey. Making less aggressive load
> balance in the wake up path would probably reduce performance for the
> type of workload you quoted earlier and for db workload, we don't want
> any of them at all, not even the code to determine whether it should
> be balanced or not.

i think we could try to get rid of wakeup-time balancing altogether.

these days pretty much the only time we can sensibly do 'fast' (as in
immediate) migration are fork/clone and exec. Furthermore, the gained
simplicity of wakeup is quite compelling too. (Originally, when i
introduced the first variant wakeup-time balancing eons ago, we didnt
have anything like fork-time and exec-time balancing.)

i think we could try the patch below in -mm, it removes (non-)affine
wakeup and passive wakeup-balancing, but keeps SD_WAKE_IDLE that is
needed for efficient SMT scheduling. I test-booted the patch on x86, and
it should work on all architectures. (I have tested various local-IPC
and non-IPC workloads and only found performance improvements - but i'm
sure regressions exist too, and need to be examined.)

Ingo

------
remove wakeup-time balancing. It turns out exec-time and fork-time
balancing combined with periodic rebalancing ticks does a good enough
job.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-i386/topology.h | 3 -
include/asm-ia64/topology.h | 6 --
include/asm-mips/mach-ip27/topology.h | 3 -
include/asm-ppc64/topology.h | 3 -
include/asm-x86_64/topology.h | 3 -
include/linux/sched.h | 4 -
include/linux/topology.h | 4 -
kernel/sched.c | 89 +++-------------------------------
8 files changed, 16 insertions(+), 99 deletions(-)

Index: linux-prefetch-task/include/asm-i386/topology.h
===================================================================
--- linux-prefetch-task.orig/include/asm-i386/topology.h
+++ linux-prefetch-task/include/asm-i386/topology.h
@@ -81,8 +81,7 @@ static inline int node_to_first_cpu(int
.per_cpu_gain = 100, \
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_EXEC \
- | SD_BALANCE_FORK \
- | SD_WAKE_BALANCE, \
+ | SD_BALANCE_FORK, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/include/asm-ia64/topology.h
===================================================================
--- linux-prefetch-task.orig/include/asm-ia64/topology.h
+++ linux-prefetch-task/include/asm-ia64/topology.h
@@ -65,8 +65,7 @@ void build_cpu_to_node_map(void);
.forkexec_idx = 1, \
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_NEWIDLE \
- | SD_BALANCE_EXEC \
- | SD_WAKE_AFFINE, \
+ | SD_BALANCE_EXEC, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
@@ -91,8 +90,7 @@ void build_cpu_to_node_map(void);
.per_cpu_gain = 100, \
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_EXEC \
- | SD_BALANCE_FORK \
- | SD_WAKE_BALANCE, \
+ | SD_BALANCE_FORK, \
.last_balance = jiffies, \
.balance_interval = 64, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/include/asm-mips/mach-ip27/topology.h
===================================================================
--- linux-prefetch-task.orig/include/asm-mips/mach-ip27/topology.h
+++ linux-prefetch-task/include/asm-mips/mach-ip27/topology.h
@@ -28,8 +28,7 @@ extern unsigned char __node_distances[MA
.cache_nice_tries = 1, \
.per_cpu_gain = 100, \
.flags = SD_LOAD_BALANCE \
- | SD_BALANCE_EXEC \
- | SD_WAKE_BALANCE, \
+ | SD_BALANCE_EXEC, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/include/asm-ppc64/topology.h
===================================================================
--- linux-prefetch-task.orig/include/asm-ppc64/topology.h
+++ linux-prefetch-task/include/asm-ppc64/topology.h
@@ -52,8 +52,7 @@ static inline int node_to_first_cpu(int
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_EXEC \
| SD_BALANCE_NEWIDLE \
- | SD_WAKE_IDLE \
- | SD_WAKE_BALANCE, \
+ | SD_WAKE_IDLE, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/include/asm-x86_64/topology.h
===================================================================
--- linux-prefetch-task.orig/include/asm-x86_64/topology.h
+++ linux-prefetch-task/include/asm-x86_64/topology.h
@@ -48,8 +48,7 @@ extern int __node_distance(int, int);
.per_cpu_gain = 100, \
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_FORK \
- | SD_BALANCE_EXEC \
- | SD_WAKE_BALANCE, \
+ | SD_BALANCE_EXEC, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/include/linux/sched.h
===================================================================
--- linux-prefetch-task.orig/include/linux/sched.h
+++ linux-prefetch-task/include/linux/sched.h
@@ -471,9 +471,7 @@ enum idle_type
#define SD_BALANCE_EXEC 4 /* Balance on exec */
#define SD_BALANCE_FORK 8 /* Balance on fork, clone */
#define SD_WAKE_IDLE 16 /* Wake to idle CPU on task wakeup */
-#define SD_WAKE_AFFINE 32 /* Wake task to waking CPU */
-#define SD_WAKE_BALANCE 64 /* Perform balancing at task wakeup */
-#define SD_SHARE_CPUPOWER 128 /* Domain members share cpu power */
+#define SD_SHARE_CPUPOWER 32 /* Domain members share cpu power */

struct sched_group {
struct sched_group *next; /* Must be a circular list */
Index: linux-prefetch-task/include/linux/topology.h
===================================================================
--- linux-prefetch-task.orig/include/linux/topology.h
+++ linux-prefetch-task/include/linux/topology.h
@@ -97,7 +97,6 @@
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_NEWIDLE \
| SD_BALANCE_EXEC \
- | SD_WAKE_AFFINE \
| SD_WAKE_IDLE \
| SD_SHARE_CPUPOWER, \
.last_balance = jiffies, \
@@ -127,8 +126,7 @@
.forkexec_idx = 1, \
.flags = SD_LOAD_BALANCE \
| SD_BALANCE_NEWIDLE \
- | SD_BALANCE_EXEC \
- | SD_WAKE_AFFINE, \
+ | SD_BALANCE_EXEC, \
.last_balance = jiffies, \
.balance_interval = 1, \
.nr_balance_failed = 0, \
Index: linux-prefetch-task/kernel/sched.c
===================================================================
--- linux-prefetch-task.orig/kernel/sched.c
+++ linux-prefetch-task/kernel/sched.c
@@ -254,7 +254,6 @@ struct runqueue {

/* try_to_wake_up() stats */
unsigned long ttwu_cnt;
- unsigned long ttwu_local;
#endif
};

@@ -373,7 +372,7 @@ static inline void task_rq_unlock(runque
* bump this up when changing the output format or the meaning of an existing
* format, so that tools can adapt (or abort)
*/
-#define SCHEDSTAT_VERSION 12
+#define SCHEDSTAT_VERSION 13

static int show_schedstat(struct seq_file *seq, void *v)
{
@@ -390,11 +389,11 @@ static int show_schedstat(struct seq_fil

/* runqueue-specific stats */
seq_printf(seq,
- "cpu%d %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu",
+ "cpu%d %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu",
cpu, rq->yld_both_empty,
rq->yld_act_empty, rq->yld_exp_empty, rq->yld_cnt,
rq->sched_switch, rq->sched_cnt, rq->sched_goidle,
- rq->ttwu_cnt, rq->ttwu_local,
+ rq->ttwu_cnt,
rq->rq_sched_info.cpu_time,
rq->rq_sched_info.run_delay, rq->rq_sched_info.pcnt);

@@ -424,8 +423,7 @@ static int show_schedstat(struct seq_fil
seq_printf(seq, " %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu\n",
sd->alb_cnt, sd->alb_failed, sd->alb_pushed,
sd->sbe_cnt, sd->sbe_balanced, sd->sbe_pushed,
- sd->sbf_cnt, sd->sbf_balanced, sd->sbf_pushed,
- sd->ttwu_wake_remote, sd->ttwu_move_affine, sd->ttwu_move_balance);
+ sd->sbf_cnt, sd->sbf_balanced, sd->sbf_pushed);
}
preempt_enable();
#endif
@@ -1134,8 +1132,6 @@ static int try_to_wake_up(task_t * p, un
long old_state;
runqueue_t *rq;
#ifdef CONFIG_SMP
- unsigned long load, this_load;
- struct sched_domain *sd, *this_sd = NULL;
int new_cpu;
#endif

@@ -1154,77 +1150,13 @@ static int try_to_wake_up(task_t * p, un
if (unlikely(task_running(rq, p)))
goto out_activate;

- new_cpu = cpu;
-
schedstat_inc(rq, ttwu_cnt);
- if (cpu == this_cpu) {
- schedstat_inc(rq, ttwu_local);
- goto out_set_cpu;
- }
-
- for_each_domain(this_cpu, sd) {
- if (cpu_isset(cpu, sd->span)) {
- schedstat_inc(sd, ttwu_wake_remote);
- this_sd = sd;
- break;
- }
- }
-
- if (unlikely(!cpu_isset(this_cpu, p->cpus_allowed)))
- goto out_set_cpu;

/*
- * Check for affine wakeup and passive balancing possibilities.
+ * Wake to the CPU the task was last running on (or any
+ * nearby SMT-equivalent idle CPU):
*/
- if (this_sd) {
- int idx = this_sd->wake_idx;
- unsigned int imbalance;
-
- imbalance = 100 + (this_sd->imbalance_pct - 100) / 2;
-
- load = source_load(cpu, idx);
- this_load = target_load(this_cpu, idx);
-
- new_cpu = this_cpu; /* Wake to this CPU if we can */
-
- if (this_sd->flags & SD_WAKE_AFFINE) {
- unsigned long tl = this_load;
- /*
- * If sync wakeup then subtract the (maximum possible)
- * effect of the currently running task from the load
- * of the current CPU:
- */
- if (sync)
- tl -= SCHED_LOAD_SCALE;
-
- if ((tl <= load &&
- tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
- 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
- /*
- * This domain has SD_WAKE_AFFINE and
- * p is cache cold in this domain, and
- * there is no bad imbalance.
- */
- schedstat_inc(this_sd, ttwu_move_affine);
- goto out_set_cpu;
- }
- }
-
- /*
- * Start passive balancing when half the imbalance_pct
- * limit is reached.
- */
- if (this_sd->flags & SD_WAKE_BALANCE) {
- if (imbalance*this_load <= 100*load) {
- schedstat_inc(this_sd, ttwu_move_balance);
- goto out_set_cpu;
- }
- }
- }
-
- new_cpu = cpu; /* Could not wake to this_cpu. Wake to cpu instead */
-out_set_cpu:
- new_cpu = wake_idle(new_cpu, p);
+ new_cpu = wake_idle(cpu, p);
if (new_cpu != cpu) {
set_task_cpu(p, new_cpu);
task_rq_unlock(rq, &flags);
@@ -4758,9 +4690,7 @@ static int sd_degenerate(struct sched_do
}

/* Following flags don't use groups */
- if (sd->flags & (SD_WAKE_IDLE |
- SD_WAKE_AFFINE |
- SD_WAKE_BALANCE))
+ if (sd->flags & SD_WAKE_IDLE)
return 0;

return 1;
@@ -4778,9 +4708,6 @@ static int sd_parent_degenerate(struct s
return 0;

/* Does parent contain flags not in child? */
- /* WAKE_BALANCE is a subset of WAKE_AFFINE */
- if (cflags & SD_WAKE_AFFINE)
- pflags &= ~SD_WAKE_BALANCE;
/* Flags needing groups don't count if only 1 group in parent */
if (parent->groups == parent->groups->next) {
pflags &= ~(SD_LOAD_BALANCE |
-
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