On Fri, May 31, 2019 at 03:12:13PM -0600, Jens Axboe wrote:
> On 5/30/19 2:03 AM, Peter Zijlstra wrote:
> > What is the purpose of that patch ?! The Changelog doesn't mention any
> > benefit or performance gain. So why not revert that?
>
> Yeah that is actually pretty weak. There are substantial performance
> gains for small IOs using this trick, the changelog should have
> included those. I guess that was left on the list...
OK. I've looked at the try_to_wake_up() path for these exact
conditions and we're certainly sub-optimal there, and I think we can put
much of this special case in there. Please see below.
> I know it's not super kosher, your patch, but I don't think it's that
> bad hidden in a generic helper.
How about the thing that Oleg proposed? That is, not set a waiter when
we know the loop is polling? That would avoid the need for this
alltogether, it would also avoid any set_current_state() on the wait
side of things.
Anyway, Oleg, do you see anything blatantly buggered with this patch?
(the stats were already dodgy for rq-stats, this patch makes them dodgy
for task-stats too)
---
kernel/sched/core.c | 38 ++++++++++++++++++++++++++++++++------
1 file changed, 32 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 102dfcf0a29a..474aa4c8e9d2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1990,6 +1990,28 @@ try_to_wake_up(struct task_struct *p, unsigned int
state, int wake_flags)
unsigned long flags;
int cpu, success = 0;
+ if (p == current) {
+ /*
+ * We're waking current, this means 'p->on_rq' and 'task_cpu(p)
+ * == smp_processor_id()'. Together this means we can special
+ * case the whole 'p->on_rq && ttwu_remote()' case below
+ * without taking any locks.
+ *
+ * In particular:
+ * - we rely on Program-Order guarantees for all the ordering,
+ * - we're serialized against set_special_state() by virtue of
+ * it disabling IRQs (this allows not taking ->pi_lock).
+ */
+ if (!(p->state & state))
+ goto out;
+
+ success = 1;
+ trace_sched_waking(p);
+ p->state = TASK_RUNNING;
+ trace_sched_woken(p);
+ goto out;
+ }
+
/*
* If we are going to wake up a thread waiting for CONDITION we
* need to ensure that CONDITION=1 done by the caller can not be
@@ -1999,7 +2021,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state,
int wake_flags)
raw_spin_lock_irqsave(&p->pi_lock, flags);
smp_mb__after_spinlock();
if (!(p->state & state))
- goto out;
+ goto unlock;
trace_sched_waking(p);
@@ -2029,7 +2051,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state,
int wake_flags)
*/
smp_rmb();
if (p->on_rq && ttwu_remote(p, wake_flags))
- goto stat;
+ goto unlock;
#ifdef CONFIG_SMP
/*
@@ -2089,12 +2111,16 @@ try_to_wake_up(struct task_struct *p, unsigned int
state, int wake_flags)
#endif /* CONFIG_SMP */
ttwu_queue(p, cpu, wake_flags);
-stat:
- ttwu_stat(p, cpu, wake_flags);
-out:
+unlock:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
- return success;
+out:
+ if (success) {
+ ttwu_stat(p, cpu, wake_flags);
+ return true;
+ }
+
+ return false;
}
/**
