Message-ID: <bf8689c2-0749-47cb-9535-53cf66e34f5e@paulmck-laptop>
Date: Thu, 21 Mar 2024 05:47:59 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [PATCH 2/2] timers: Fix removed self-IPI on global timer's
 enqueue in nohz_full

On Thu, Mar 21, 2024 at 12:42:09PM +0100, Frederic Weisbecker wrote:
> On Wed, Mar 20, 2024 at 03:55:17PM -0700, Paul E. McKenney wrote:
> > On Wed, Mar 20, 2024 at 05:15:48PM +0100, Frederic Weisbecker wrote:
> > > Le Wed, Mar 20, 2024 at 04:14:24AM -0700, Paul E. McKenney a écrit :
> > > > On Tue, Mar 19, 2024 at 02:18:00AM -0700, Paul E. McKenney wrote:
> > > > > On Tue, Mar 19, 2024 at 12:07:29AM +0100, Frederic Weisbecker wrote:
> > > > > > While running in nohz_full mode, a task may enqueue a timer while the
> > > > > > tick is stopped. However the only places where the timer wheel,
> > > > > > alongside the timer migration machinery's decision, may reprogram the
> > > > > > next event accordingly with that new timer's expiry are the idle loop or
> > > > > > any IRQ tail.
> > > > > > 
> > > > > > However neither the idle task nor an interrupt may run on the CPU if it
> > > > > > resumes busy work in userspace for a long while in full dynticks mode.
> > > > > > 
> > > > > > To solve this, the timer enqueue path raises a self-IPI that will
> > > > > > re-evaluate the timer wheel on its IRQ tail. This asynchronous solution
> > > > > > avoids potential locking inversion.
> > > > > > 
> > > > > > This is supposed to happen both for local and global timers but commit:
> > > > > > 
> > > > > > 	b2cf7507e186 ("timers: Always queue timers on the local CPU")
> > > > > > 
> > > > > > broke the global timers case with removing the ->is_idle field handling
> > > > > > for the global base. As a result, global timers enqueue may go unnoticed
> > > > > > in nohz_full.
> > > > > > 
> > > > > > Fix this with restoring the idle tracking of the global timer's base,
> > > > > > allowing self-IPIs again on enqueue time.
> > > > > 
> > > > > Testing with the previous patch (1/2 in this series) reduced the number of
> > > > > problems by about an order of magnitude, down to two sched_tick_remote()
> > > > > instances and one enqueue_hrtimer() instance, very good!
> > > > > 
> > > > > I have kicked off a test including this patch.  Here is hoping!  ;-)
> > > > 
> > > > And 22*100 hours of TREE07 got me one run with a sched_tick_remote()
> > 
> > Sigh.  s/22/12/
> > 
> > > > complaint and another run with a starved RCU grace-period kthread.
> > > > So this is definitely getting more reliable, but still a little ways
> > > > to go.
> > 
> > An additional eight hours got anohtre sched_tick_remote().
> > 
> > > Right, there is clearly something else. Investigation continues...
> > 
> > Please let me know if there is a debug patch that I could apply.
> 
> So there are three things:
> 
> _ The sched_tick_remote() warning. I can easily trigger this one and while
>   trying a bisection, I realize it actually also triggers on v6.8
>   I'm not really tempted to investigate further because the warning doesn't make
>   much sense to me. This computes the delta between the time the kworker got
>   scheduled and the time it reaches the middle of the work. It happens to be
>   ~3s but isn't it something to be expected with jitter and all involved into
>   rcutorture?
> 
>   We should probably just remove this warning. This remote tick is my most horrible
>   hackery anyway.

Would it make sense to increase it to somewhere around the timeout for
RCU CPU stall warnings, softlockup, and so on?

> _ The hrtimer enqueue to an offline CPU warning:
> 
> 	[ 1054.265335] WARNING: CPU: 8 PID: 150 at kernel/time/hrtimer.c:1091 enqueue_hrtimer+0x6f/0x80
> 	[ 1054.269166] Modules linked in:
> 	[ 1054.270077] CPU: 8 PID: 150 Comm: rcu_torture_rea Not tainted 6.8.0-11407-ge990136580ab-dirty #21
> 	[ 1054.272768] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014
> 	[ 1054.275893] RIP: 0010:enqueue_hrtimer+0x6f/0x80
> 	[ 1054.277252] Code: a3 05 b5 ff b7 01 73 bd 48 8b 05 44 c5 b5 01 48 85 c0 74 0c 48 8b 78 08 48 89 ee e8 7b b5 ff ff 48 8b 03 f6 40 10 10 75 a5 90 <0f> 0b 90 eb 9f 66 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90
> 	[ 1054.282448] RSP: 0000:ffffb1480056fd70 EFLAGS: 00010046
> 	[ 1054.283931] RAX: ffff95b7df2616c0 RBX: ffff95b7df261700 RCX: ffff95b7df2616c0
> 	[ 1054.286091] RDX: 0000000000000001 RSI: ffff95b7df261700 RDI: ffffb1480056fde0
> 	[ 1054.288095] RBP: ffffb1480056fde0 R08: 0000000000000001 R09: 000000000000fc03
> 	[ 1054.290189] R10: 0000000000000001 R11: ffff95b7c1271c80 R12: 0000000000000040
> 	[ 1054.292592] R13: ffff95b7df261700 R14: ffff95b7df261700 R15: ffff95b7df2616c0
> 	[ 1054.294622] FS:  0000000000000000(0000) GS:ffff95b7df400000(0000) knlGS:0000000000000000
> 	[ 1054.296884] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> 	[ 1054.298497] CR2: 0000000000000000 CR3: 000000001822c000 CR4: 00000000000006f0
> 	[ 1054.300475] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> 	[ 1054.302516] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 	[ 1054.304580] Call Trace:
> 	[ 1054.305354]  <TASK>
> 	[ 1054.306009]  ? __warn+0x77/0x120
> 	[ 1054.306973]  ? enqueue_hrtimer+0x6f/0x80
> 	[ 1054.308177]  ? report_bug+0xf1/0x1d0
> 	[ 1054.310200]  ? handle_bug+0x40/0x70
> 	[ 1054.311252]  ? exc_invalid_op+0x13/0x70
> 	[ 1054.312437]  ? asm_exc_invalid_op+0x16/0x20
> 	[ 1054.313738]  ? enqueue_hrtimer+0x6f/0x80
> 	[ 1054.314994]  hrtimer_start_range_ns+0x258/0x2f0
> 	[ 1054.316531]  ? __pfx_rcu_torture_reader+0x10/0x10
> 	[ 1054.317983]  schedule_hrtimeout_range_clock+0x95/0x120
> 	[ 1054.319604]  ? __pfx_hrtimer_wakeup+0x10/0x10
> 	[ 1054.320957]  torture_hrtimeout_ns+0x50/0x70
> 	[ 1054.322211]  rcu_torture_reader+0x1be/0x280
> 	[ 1054.323455]  ? __pfx_rcu_torture_timer+0x10/0x10
> 	[ 1054.329888]  ? kthread+0xc4/0xf0
> 	[ 1054.330880]  ? __pfx_rcu_torture_reader+0x10/0x10
> 	[ 1054.332334]  kthread+0xc4/0xf0
> 	[ 1054.333305]  ? __pfx_kthread+0x10/0x10
> 	[ 1054.334466]  ret_from_fork+0x2b/0x40
> 	[ 1054.335551]  ? __pfx_kthread+0x10/0x10
> 	[ 1054.336690]  ret_from_fork_asm+0x1a/0x30
> 	[ 1054.337878]  </TASK>
> 
>   I don't know how it happened. This is where I'm investigating now.

It seems that rcutorture is the gift that keeps on giving?  ;-)

> _ The RCU CPU Stall report. I strongly suspect the cause is the hrtimer
>   enqueue to an offline CPU. Let's solve that and we'll see if it still
>   triggers.

Sounds like a plan!

							Thanx, Paul

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