Re: PostgreSQL pgbench performance regression in 2.6.23+

Peter Zijlstra
2008-05-22T12:25:30Z

On Thu, 2008-05-22 at 14:09 +0200, Mike Galbraith wrote:
> On Thu, 2008-05-22 at 13:44 +0200, Peter Zijlstra wrote:
>
> > Humm,.. how to fix this.. we'd need to somehow detect the 1:n nature of
> > its operation - I'm sure there are other scenarios that could benefit
> > from this.
>
> Maybe simple (minded): cache waker's last non-interrupt context wakee,
> if the wakee != cached, ignore SYNC_WAKEUP unless sync was requested at
> call time?

Yeah, something like so - or perhaps like you say cache the wakee.

I picked the wake_affine() condition, because I think that is the
biggest factor in this behaviour. You could of course also disable all
of sync.

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c86c5c5..856c2a8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -950,6 +950,8 @@ struct sched_entity {
u64 last_wakeup;
u64 avg_overlap;

+ struct sched_entity *waker;
+
#ifdef CONFIG_SCHEDSTATS
u64 wait_start;
u64 wait_max;
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 894a702..8971044 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1036,7 +1036,8 @@ wake_affine(struct rq *rq, struct sched_domain *this_sd, struct rq *this_rq,
* a reasonable amount of time then attract this newly
* woken task:
*/
- if (sync && curr->sched_class == &fair_sched_class) {
+ if (sync && curr->sched_class == &fair_sched_class &&
+ p->se.waker == curr->se->waker) {
if (curr->se.avg_overlap < sysctl_sched_migration_cost &&
p->se.avg_overlap < sysctl_sched_migration_cost)
return 1;
@@ -1210,6 +1211,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p)
if (unlikely(se == pse))
return;

+ se->waker = pse;
cfs_rq_of(pse)->next = pse;

/*