linux-kernel - Re: [tip:timers/urgent] timers: Unconditionally check deferrable base

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Date:   Mon, 15 Jan 2018 15:57:37 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     bigeasy@...utronix.de, fweisbec@...il.com,
        linux-kernel@...r.kernel.org, hpa@...or.com, peterz@...radead.org,
        mingo@...nel.org, tglx@...utronix.de, anna-maria@...utronix.de
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/urgent] timers: Unconditionally check deferrable base

On Sun, Jan 14, 2018 at 02:44:15PM -0800, Paul E. McKenney wrote:
> On Sun, Jan 14, 2018 at 02:31:07PM -0800, tip-bot for Thomas Gleixner wrote:
> > Commit-ID:  ed4bbf7910b28ce3c691aef28d245585eaabda06
> > Gitweb:     https://git.kernel.org/tip/ed4bbf7910b28ce3c691aef28d245585eaabda06
> > Author:     Thomas Gleixner <tglx@...utronix.de>
> > AuthorDate: Sun, 14 Jan 2018 23:19:49 +0100
> > Committer:  Thomas Gleixner <tglx@...utronix.de>
> > CommitDate: Sun, 14 Jan 2018 23:25:33 +0100
> > 
> > timers: Unconditionally check deferrable base
> > 
> > When the timer base is checked for expired timers then the deferrable base
> > must be checked as well. This was missed when making the deferrable base
> > independent of base::nohz_active.
> 
> Very cool, thank you!  I have fired up some of the dreaded TREE01 tests.

And (tentatively) a change in behavior!

In one case, I did not see stall warnings (at least not until the ftrace
dump at test end), but just the occasional "Waylayed timer" message:

[ 1316.628105] schedule_timeout: Waylayed timer base->clk: 0x1000eff23 jiffies: 0x1000f8335 base->next_expiry: 0x1000eff23 timer->flags: 0x3c800006 timer->expires 0x1000f62ca idx: f2 idx_now: 139 base->pending_map 000000080000000000000000000000000000000000000000100400002000000000000000000000000000010000000000000000000000000000000000000000000000000000000000
[ 1316.638372] Torture onoff task state:
[ 1316.639774] rcu_preempt     I14984     8      2 0x80000000
[ 1316.641766] Call Trace:
[ 1316.642757]  ? __schedule+0x33c/0x6f0
[ 1316.644109]  ? trace_event_raw_event_timer_start+0x6d/0xb0
[ 1316.646042]  schedule+0x37/0x90
[ 1316.647219]  schedule_timeout+0x159/0x470
[ 1316.648825]  ? __next_timer_interrupt+0xc0/0xc0
[ 1316.650459]  rcu_gp_kthread+0x9ee/0x1090
[ 1316.652147]  kthread+0xf0/0x130
[ 1316.653415]  ? rcu_oom_notify+0xd0/0xd0
[ 1316.654790]  ? kthread_destroy_worker+0x40/0x40
[ 1316.656470]  ret_from_fork+0x1f/0x30

If I am reading the waylay message correctly, the timer was delayed by
about 8.3 seconds instead of the 21 seconds required to cause an RCU
CPU stall warning.  I am going to rearrange the ftrace-dump code a bit
and run more tests.

The next one got the usual stall warning and:

[  633.772036] schedule_timeout: Waylayed timer base->clk: 0x10004c5cb jiffies: 0x10005181e base->next_expiry: 0x10004c5cb timer->flags: 0x2c00006 timer->expires 0x10004c5ca idx: b idx_now: cd base->pending_map 000000000000080000000000000000000000020000000000000020000000000000000000000000000000000000080000000000000000000000000000000000000000000000000000
[  633.855300] Torture onoff task state:
[  633.855300] rcu_preempt     I14984     8      2 0x80000000
[  633.855300] Call Trace:
[  633.855300]  ? __schedule+0x33c/0x6f0
[  633.855300]  ? _raw_spin_lock_irqsave+0x12/0x40 
[  633.855300]  schedule+0x37/0x90
[  633.855300]  rcu_gp_kthread+0xad/0x1090
[  633.855300]  kthread+0xf0/0x130
[  633.855300]  ? rcu_oom_notify+0xd0/0xd0
[  633.855300]  ? kthread_destroy_worker+0x40/0x40
[  633.855300]  ret_from_fork+0x1f/0x30

And the third one also got the usual stall warning and:

[  382.482240] schedule_timeout: Waylayed timer base->clk: 0x10000322a jiffies: 0x100014232 base->next_expiry: 0x10000322a timer->flags: 0x44000001 timer->expires 0x10000f00f idx: 110 idx_now: 115 base->pending_map 000004000000000000000000000000000000000000000000000000000100000000000000000140000000000000000400000000000000000000000000000000000000000000000000
[  382.482245] Torture onoff task state:
[  382.482246] rcu_preempt     I14984     8      2 0x80000000
[  382.482273] Call Trace:
[  382.482280]  ? __schedule+0x33c/0x6f0
[  382.482283]  ? trace_event_raw_event_timer_start+0x6d/0xb0
[  382.482285]  schedule+0x37/0x90
[  382.482287]  schedule_timeout+0x159/0x470
[  382.482289]  ? __next_timer_interrupt+0xc0/0xc0
[  382.482292]  rcu_gp_kthread+0x9ee/0x1090
[  382.482297]  kthread+0xf0/0x130
[  382.482299]  ? rcu_oom_notify+0xd0/0xd0
[  382.482301]  ? kthread_destroy_worker+0x40/0x40
[  382.482303]  ret_from_fork+0x1f/0x30

I don't have enough runs to be sure, but my current (sad) guess is that
the error rate is unchanged.

							Thanx, Paul