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Date: Mon, 23 Apr 2018 11:54:20 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Niklas Söderlund
<niklas.soderlund@...natech.se>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...hat.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-renesas-soc@...r.kernel.org
Subject: Re: Potential problem with 31e77c93e432dec7 ("sched/fair: Update
blocked load when newly idle")
Hi Niklas,
Le Monday 23 Apr 2018 à 00:18:27 (+0200), Niklas Söderlund a écrit :
> Hi Vincent,
>
> On 2018-04-20 18:00:13 +0200, Vincent Guittot wrote:
>
> [snip]
>
> > >
> > > You results are similar to Heiner's ones. The problem is still there
> > > even if we only kick ilb which mainly send an IPI reschedule to the
> > > other CPU if Idle
> > >
> >
> > Could it be possible to record some traces of the problem to get a better view
> > of what happens ?
> >
> > I have a small patch that adds some traces in the functions that seems to create
> > the problem
>
> Sure, I applied the patch bellow on-top of c18bb396d3d261eb ("Merge
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net").
>
> >
> >
> >
> > Also that would be good to stop tracing when the RCU stall happens
> >
> > In case you are not familiar with the trace tool, I have put below how to configure ftrace to trace scheudler, irq, timer ... events and stop tracing when dump_backtrace function is called
> >
> > trace-cmd reset > /dev/null
> > trace-cmd start -b 40000 -p function -l dump_backtrace:traceoff -e sched -e cpu_idle -e cpu_frequency -e timer -e ipi -e irq -e printk
> > trace-cmd start -b 40000 -p function -l dump_backtrace
> >
> > The trace-cmd start has to be called twice to make sure that we will not trace all function
> >
> > Once the dump happen and the trace is stopped, you can extract the traces with
> >
> > trace-cmd extract -o <trace file name>
>
> Thanks for the help with trace-cmd, I have attached the full
> trace-cmd.dat extracted from running this. But a quick run of trace-cmd
> report.i
Thanks for the report. Can you re run with the following trace-cmd sequence ? My previous sequence disables ftrace events
trace-cmd reset > /dev/null
trace-cmd start -b 40000 -p function -l dump_backtrace:traceoff -e sched -e cpu_idle -e cpu_frequency -e timer -e ipi -e irq -e printk
trace-cmd start -b 40000 -p function -l dump_backtrace -e sched -e cpu_idle -e cpu_frequency -e timer -e ipi -e irq -e printk
I have updated the patch and added traces to check that scheduler returns from idle_balance function and doesn't stay stuck
---
kernel/sched/fair.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0951d1c..4285511 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9606,6 +9606,8 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
*/
WRITE_ONCE(nohz.has_blocked, 0);
+ trace_printk("_nohz_idle_balance cpu %d idle %d flag %x", this_cpu, idle, flags);
+
/*
* Ensures that if we miss the CPU, we must see the has_blocked
* store from nohz_balance_enter_idle().
@@ -9628,6 +9630,8 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
rq = cpu_rq(balance_cpu);
+ trace_printk("_nohz_idle_balance update cpu %d ", balance_cpu);
+
has_blocked_load |= update_nohz_stats(rq, true);
/*
@@ -9680,6 +9684,8 @@ static bool _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
if (likely(update_next_balance))
nohz.next_balance = next_balance;
+ trace_printk("_nohz_idle_balance return %d", ret);
+
return ret;
}
@@ -9732,6 +9738,8 @@ static void nohz_newidle_balance(struct rq *this_rq)
time_before(jiffies, READ_ONCE(nohz.next_blocked)))
return;
+ trace_printk("nohz_newidle_balance start update");
+
raw_spin_unlock(&this_rq->lock);
/*
* This CPU is going to be idle and blocked load of idle CPUs
@@ -9742,6 +9750,9 @@ static void nohz_newidle_balance(struct rq *this_rq)
if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
kick_ilb(NOHZ_STATS_KICK);
raw_spin_lock(&this_rq->lock);
+
+ trace_printk("nohz_newidle_balance lock back");
+
}
#else /* !CONFIG_NO_HZ_COMMON */
@@ -9869,6 +9880,8 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
rq_repin_lock(this_rq, rf);
+ trace_printk("idle_balance %d", pulled_task);
+
return pulled_task;
}
--
2.7.4
>
>
[snip]
> rcu_sched-9 [000] 147.342741: bputs: pick_next_task_fair: nohz_newidle_balance start update
> rcu_sched-9 [000] 147.342749: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 0 idle 2 flag 2
> rcu_sched-9 [000] 147.342754: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> rcu_sched-9 [000] 147.382739: bputs: pick_next_task_fair: nohz_newidle_balance start update
> rcu_sched-9 [000] 147.382741: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 0 idle 2 flag 2
> rcu_sched-9 [000] 147.382745: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> sshd-1881 [000] 147.621105: bputs: pick_next_task_fair: nohz_newidle_balance start update
> sshd-1881 [000] 147.621108: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 0 idle 2 flag 2
> sshd-1881 [000] 147.621110: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
This is the last call to the function added by the patch on CPU0 and it returns completely.
But then nothing else is schedule on CPU0
> <idle>-0 [001] 147.662725: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 0 flag 2
> <idle>-0 [001] 147.662730: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 151.032830: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 151.032833: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 151.032835: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 155.032777: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 155.032780: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 155.032781: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> haveged-1823 [001] 155.515789: bputs: pick_next_task_fair: nohz_newidle_balance start update
> haveged-1823 [001] 155.515792: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> haveged-1823 [001] 155.515794: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 157.032718: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 157.032720: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 157.032722: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 159.032724: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 159.032726: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 159.032728: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 161.032698: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 161.032701: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 161.032702: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 165.032671: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 165.032673: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 165.032675: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 167.032667: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 167.032669: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 167.032671: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 169.032654: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 169.032657: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 169.032658: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> rpcbind-1847 [001] 169.472688: bputs: pick_next_task_fair: nohz_newidle_balance start update
> rpcbind-1847 [001] 169.472691: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> rpcbind-1847 [001] 169.472693: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> kworker/1:1-31 [001] 171.032660: bputs: pick_next_task_fair: nohz_newidle_balance start update
> kworker/1:1-31 [001] 171.032662: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> kworker/1:1-31 [001] 171.032664: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> rpcbind-1847 [001] 199.482558: bputs: pick_next_task_fair: nohz_newidle_balance start update
> rpcbind-1847 [001] 199.482561: bprint: _nohz_idle_balance: _nohz_idle_balance cpu 1 idle 2 flag 2
> rpcbind-1847 [001] 199.482563: bprint: _nohz_idle_balance: _nohz_idle_balance return 1
> <idle>-0 [000] 241.615152: function: dump_backtrace
>
> --
> Regards,
> Niklas Söderlund
sorry for the inconvenience
Vincent
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