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SubjectRe: Scheduler bug related to rq->skip_clock_update?
On Mon, 22 Nov 2010, Mike Galbraith wrote:

> On Sun, 2010-11-21 at 23:29 -0500, Bjoern B. Brandenburg wrote:
> > On Sun, 21 Nov 2010, Mike Galbraith wrote:
> >
> > > On Sat, 2010-11-20 at 23:22 -0500, Bjoern B. Brandenburg wrote:
> > >
> > > > I was under the impression that, as an invariant, tasks should not have
> > > > TIF_NEED_RESCHED set after they've blocked. In this case, the idle load
> > > > balancer should not mark the task that's on its way out with
> > > > set_tsk_need_resched().
> > >
> > > Nice find.
> > >
> > > > In any case, check_preempt_curr() seems to assume that a resuming task cannot
> > > > have TIF_NEED_RESCHED already set. Setting skip_clock_update on a remote CPU
> > > > that hasn't even been notified via IPI seems wrong.
> > >
> > > Yes. Does the below fix it up for you?
> >
> > The patch definitely changes the behavior, but it doesn't seem to solve (all
> > of) the root cause(s). The failsafe kicks in and clears the flag the next
> > time that update_rq_clock() is called, but there can still be a significant
> > delay between setting and clearing the flag. Right after boot, I'm now seeing
> > values that go up to ~21ms.
>
> A pull isn't the only vulnerability. Since idle_balance() drops
> rq->lock, so another cpu can wake to this rq.
>
> > Please let me know if there is something else that I should test.
>
> Sched: clear_tsk_need_resched() after NEWIDLE balancing
>
> idle_balance() drops/retakes rq->lock, leaving the previous task
> vulnerable to set_tsk_need_resched() from another CPU. Clear it
> after NEWIDLE balancing to maintain the invariant that descheduled
> tasks are NOT marked for resched.
>
> This also confuses the skip_clock_update logic, which assumes that
> the next call to update_rq_clock() will come nearly ĩmmediately after
> being set. Make the optimization more robust by clearing before we
> balance and in update_rq_clock().

Unfortunately that doesn't seem to do it yet.

After running five 'find /' instances to completion on the ARM platform,
I'm still seeing delays close to 10ms.

bbb@district10:~$ egrep 'cpu#|skip' /proc/sched_debug
cpu#0
.skip_clock_count : 89606
.skip_clock_recent_max : 9817250
.skip_clock_max : 21992375
cpu#1
.skip_clock_count : 81978
.skip_clock_recent_max : 9582500
.skip_clock_max : 17201750
cpu#2
.skip_clock_count : 74565
.skip_clock_recent_max : 9678000
.skip_clock_max : 9879250
cpu#3
.skip_clock_count : 81685
.skip_clock_recent_max : 9300125
.skip_clock_max : 14115750

On the x86_64 host, I've changed to HZ=100 and am now also seeing delays
close to 10ms after 'make clean && make -j8 bzImage'.

bbb@koruna:~$ egrep 'cpu#|skip' /proc/sched_debug
cpu#0, 2493.476 MHz
.skip_clock_count : 29703
.skip_clock_recent_max : 9999858
.skip_clock_max : 40645942
cpu#1, 2493.476 MHz
.skip_clock_count : 32696
.skip_clock_recent_max : 9959118
.skip_clock_max : 35074771
cpu#2, 2493.476 MHz
.skip_clock_count : 31742
.skip_clock_recent_max : 9788654
.skip_clock_max : 24821765
cpu#3, 2493.476 MHz
.skip_clock_count : 31123
.skip_clock_recent_max : 9858546
.skip_clock_max : 44276033
cpu#4, 2493.476 MHz
.skip_clock_count : 28346
.skip_clock_recent_max : 10000775
.skip_clock_max : 18681753
cpu#5, 2493.476 MHz
.skip_clock_count : 29421
.skip_clock_recent_max : 9997656
.skip_clock_max : 138473407
cpu#6, 2493.476 MHz
.skip_clock_count : 27721
.skip_clock_recent_max : 9992074
.skip_clock_max : 53436918
cpu#7, 2493.476 MHz
.skip_clock_count : 29637
.skip_clock_recent_max : 9994516
.skip_clock_max : 566793528

These numbers were recorded with the below patch.

Please let me know if I can help by testing or tracing something else.

Thanks,
Bjoern

---
sched: show length of runqueue clock deactivation in /proc/sched_debug

The runqueue clock update should obviously not be skipped for
prolonged times (otherwise the consumed time is not correctly kept
track of). This patch measures the time between setting and clearing
the rq->skip_clock_update flag. The maximum observed value is exported
in /proc/sched_debug.
---
kernel/sched.c | 30 +++++++++++++++++++++++++++---
kernel/sched_debug.c | 4 ++++
2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 29509e1..08839ce 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -541,6 +541,11 @@ struct rq {
unsigned long long rq_cpu_time;
/* could above be rq->cfs_rq.exec_clock + rq->rt_rq.rt_runtime ? */

+ u64 skip_clock_max;
+ u64 skip_clock_recent_max;
+ u64 skip_clock_set;
+ unsigned int skip_clock_count;
+
/* sys_sched_yield() stats */
unsigned int yld_count;

@@ -639,6 +644,22 @@ static inline struct task_group *task_group(struct task_struct *p)
static u64 irq_time_cpu(int cpu);
static void sched_irq_time_avg_update(struct rq *rq, u64 irq_time);

+static void clear_skip_clock_update(struct rq *rq)
+{
+#ifdef CONFIG_SCHEDSTATS
+ if (unlikely(rq->skip_clock_update)) {
+ u64 skipped = sched_clock_cpu(cpu_of(rq)) - rq->skip_clock_set;
+ rq->skip_clock_max = max(rq->skip_clock_max, skipped);
+ /* reset infrequently to expose changes */
+ if (!(++rq->skip_clock_count % 10000))
+ rq->skip_clock_recent_max = 0;
+ rq->skip_clock_recent_max = max(rq->skip_clock_recent_max,
+ skipped);
+ }
+#endif
+ rq->skip_clock_update = 0;
+}
+
inline void update_rq_clock(struct rq *rq)
{
if (!rq->skip_clock_update) {
@@ -652,7 +673,7 @@ inline void update_rq_clock(struct rq *rq)

sched_irq_time_avg_update(rq, irq_time);
}
- rq->skip_clock_update = 0;
+ clear_skip_clock_update(rq);
}

/*
@@ -2130,8 +2151,11 @@ static void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
* A queue event has occurred, and we're going to schedule. In
* this case, we can save a useless back to back clock update.
*/
- if (test_tsk_need_resched(rq->curr))
+ if (test_tsk_need_resched(rq->curr)) {
+ schedstat_set(rq->skip_clock_set,
+ sched_clock_cpu(cpu_of(rq)));
rq->skip_clock_update = 1;
+ }
}

#ifdef CONFIG_SMP
@@ -3938,7 +3962,7 @@ need_resched_nonpreemptible:
* Clock updates should not be skipped while we're away.
*/
clear_tsk_need_resched(prev);
- rq->skip_clock_update = 0;
+ clear_skip_clock_update(rq);
}

put_prev_task(rq, prev);
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 2e1b0d1..8924be2 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -298,6 +298,10 @@ static void print_cpu(struct seq_file *m, int cpu)

P(bkl_count);

+ P(skip_clock_count);
+ P64(skip_clock_recent_max);
+ P64(skip_clock_max);
+
#undef P
#endif
print_cfs_stats(m, cpu);
--
1.7.3.1
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