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Subject[PATCH 3/3] sched: prefer wakers
Prefer tasks that wake other tasks to preempt quickly. This improves
performance because more work is available sooner.

The workload that prompted this patch was a kernel build over NFS4 (for some
curious and not understood reason we had to revert commit:
18de9735300756e3ca9c361ef58409d8561dfe0d to make any progress at all)

Without this patch a make -j8 bzImage (of x86-64 defconfig) would take
3m30-ish, with this patch we're down to 2m50-ish.

psql-sysbench/mysql-sysbench show a slight improvement in peak performance as
well, tbench and vmark seemed to not care.

It is possible to improve upon the build time (to 2m20-ish) but that seriously
destroys other benchmarks (just shows that there's more room for tinkering).

Much thanks to Mike who put in a lot of effort to benchmark things and proved
a worthy opponent with a competing patch.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
---
include/linux/sched.h | 2 +
kernel/sched.c | 30 +++++++++++++++++++++---
kernel/sched_debug.c | 1
kernel/sched_fair.c | 59 +++++++++++++++++++++++++++++++++++++++++++-----
kernel/sched_features.h | 3 +-
5 files changed, 84 insertions(+), 11 deletions(-)
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -1690,17 +1690,35 @@ static void update_avg(u64 *avg, u64 sam

static void enqueue_task(struct rq *rq, struct task_struct *p, int wakeup)
{
+ if (wakeup)
+ p->se.start_runtime = p->se.sum_exec_runtime;
+
sched_info_queued(p);
p->sched_class->enqueue_task(rq, p, wakeup);
p->se.on_rq = 1;
}

+static void update_avg_wakeup(struct sched_entity *se)
+{
+ u64 sample = se->sum_exec_runtime;
+ if (se->last_wakeup)
+ sample -= se->last_wakeup;
+ else
+ sample -= se->start_runtime;
+ update_avg(&se->avg_wakeup, sample);
+}
+
static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
- if (sleep && p->se.last_wakeup) {
- update_avg(&p->se.avg_overlap,
- p->se.sum_exec_runtime - p->se.last_wakeup);
- p->se.last_wakeup = 0;
+ if (sleep) {
+ if (p->se.last_wakeup) {
+ update_avg(&p->se.avg_overlap,
+ p->se.sum_exec_runtime - p->se.last_wakeup);
+ p->se.last_wakeup = 0;
+ } else {
+ update_avg(&p->se.avg_wakeup,
+ sysctl_sched_wakeup_granularity);
+ }
}

sched_info_dequeued(p);
@@ -2327,6 +2345,8 @@ out_activate:
schedstat_inc(p, se.nr_wakeups_remote);
activate_task(rq, p, 1);
success = 1;
+ if (!in_interrupt())
+ update_avg_wakeup(&current->se);

out_running:
trace_sched_wakeup(rq, p);
@@ -2369,6 +2389,8 @@ static void __sched_fork(struct task_str
p->se.prev_sum_exec_runtime = 0;
p->se.last_wakeup = 0;
p->se.avg_overlap = 0;
+ p->se.start_runtime = 0;
+ p->se.avg_wakeup = sysctl_sched_wakeup_granularity;

#ifdef CONFIG_SCHEDSTATS
p->se.wait_start = 0;
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -1283,16 +1283,63 @@ out:
}
#endif /* CONFIG_SMP */

-static unsigned long wakeup_gran(struct sched_entity *se)
+/*
+ * Adaptive granularity
+ *
+ * se->avg_wakeup gives the average time a task runs until it does a wakeup,
+ * with the limit of wakeup_gran -- when it never does a wakeup.
+ *
+ * So the smaller avg_wakeup is the faster we want this task to preempt,
+ * but we don't want to treat the preemptee unfairly and therefore allow it
+ * to run for at least the amount of time we'd like to run.
+ *
+ * NOTE: we use 2*avg_wakeup to increase the probability of actually doing one
+ *
+ * NOTE: we use *nr_running to scale with load, this nicely matches the
+ * degrading latency on load.
+ */
+static unsigned long
+adaptive_gran(struct sched_entity *curr, struct sched_entity *se)
+{
+ u64 this_run = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
+ u64 expected_wakeup = 2*se->avg_wakeup * cfs_rq_of(se)->nr_running;
+ u64 gran = 0;
+
+ if (this_run < expected_wakeup)
+ gran = expected_wakeup - this_run;
+
+ return min_t(s64, gran, sysctl_sched_wakeup_granularity);
+}
+
+static unsigned long
+wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
{
unsigned long gran = sysctl_sched_wakeup_granularity;

+ if (cfs_rq_of(curr)->curr && sched_feat(ADAPTIVE_GRAN))
+ gran = adaptive_gran(curr, se);
+
/*
- * More easily preempt - nice tasks, while not making it harder for
- * + nice tasks.
+ * Since its curr running now, convert the gran from real-time
+ * to virtual-time in his units.
*/
- if (!sched_feat(ASYM_GRAN) || se->load.weight > NICE_0_LOAD)
- gran = calc_delta_fair(sysctl_sched_wakeup_granularity, se);
+ if (sched_feat(ASYM_GRAN)) {
+ /*
+ * By using 'se' instead of 'curr' we penalize light tasks, so
+ * they get preempted easier. That is, if 'se' < 'curr' then
+ * the resulting gran will be larger, therefore penalizing the
+ * lighter, if otoh 'se' > 'curr' then the resulting gran will
+ * be smaller, again penalizing the lighter task.
+ *
+ * This is especially important for buddies when the leftmost
+ * task is higher priority than the buddy.
+ */
+ if (unlikely(se->load.weight != NICE_0_LOAD))
+ gran = calc_delta_fair(gran, se);
+ } else {
+ if (unlikely(curr->load.weight != NICE_0_LOAD))
+ gran = calc_delta_fair(gran, curr);
+ }

return gran;
}
@@ -1319,7 +1366,7 @@ wakeup_preempt_entity(struct sched_entit
if (vdiff <= 0)
return -1;

- gran = wakeup_gran(curr);
+ gran = wakeup_gran(curr, se);
if (vdiff > gran)
return 1;

Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1024,6 +1024,8 @@ struct sched_entity {

u64 last_wakeup;
u64 avg_overlap;
+ u64 start_runtime;
+ u64 avg_wakeup;

#ifdef CONFIG_SCHEDSTATS
u64 wait_start;
Index: linux-2.6/kernel/sched_features.h
===================================================================
--- linux-2.6.orig/kernel/sched_features.h
+++ linux-2.6/kernel/sched_features.h
@@ -1,5 +1,5 @@
SCHED_FEAT(NEW_FAIR_SLEEPERS, 1)
-SCHED_FEAT(NORMALIZED_SLEEPER, 1)
+SCHED_FEAT(NORMALIZED_SLEEPER, 0)
SCHED_FEAT(WAKEUP_PREEMPT, 1)
SCHED_FEAT(START_DEBIT, 1)
SCHED_FEAT(AFFINE_WAKEUPS, 1)
@@ -13,3 +13,4 @@ SCHED_FEAT(LB_WAKEUP_UPDATE, 1)
SCHED_FEAT(ASYM_EFF_LOAD, 1)
SCHED_FEAT(WAKEUP_OVERLAP, 0)
SCHED_FEAT(LAST_BUDDY, 1)
+SCHED_FEAT(ADAPTIVE_GRAN, 1)
Index: linux-2.6/kernel/sched_debug.c
===================================================================
--- linux-2.6.orig/kernel/sched_debug.c
+++ linux-2.6/kernel/sched_debug.c
@@ -384,6 +384,7 @@ void proc_sched_show_task(struct task_st
PN(se.vruntime);
PN(se.sum_exec_runtime);
PN(se.avg_overlap);
+ PN(se.avg_wakeup);

nr_switches = p->nvcsw + p->nivcsw;

--



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