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Date:   Mon, 8 Feb 2021 10:44:58 -0500
From:   Brian Foster <bfoster@redhat.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Paul Menzel <pmenzel@molgen.mpg.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        linux-xfs@vger.kernel.org, Josh Triplett <josh@joshtriplett.org>,
        rcu@vger.kernel.org, it+linux-rcu@molgen.mpg.de,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: rcu: INFO: rcu_sched self-detected stall on CPU: Workqueue:
 xfs-conv/md0 xfs_end_io
Message-ID: <20210208154458.GB126859@bfoster>
References: <1b07e849-cffd-db1f-f01b-2b8b45ce8c36@molgen.mpg.de>
 <20210205171240.GN2743@paulmck-ThinkPad-P72>
 <20210208140724.GA126859@bfoster>
 <20210208145723.GT2743@paulmck-ThinkPad-P72>
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On Mon, Feb 08, 2021 at 06:57:24AM -0800, Paul E. McKenney wrote:
> On Mon, Feb 08, 2021 at 09:07:24AM -0500, Brian Foster wrote:
> > On Fri, Feb 05, 2021 at 09:12:40AM -0800, Paul E. McKenney wrote:
> > > On Fri, Feb 05, 2021 at 08:29:06AM +0100, Paul Menzel wrote:
> > > > Dear Linux folks,
> > > > 
> > > > 
> > > > On a Dell PowerEdge T630/0NT78X, BIOS 2.8.0 05/23/2018 with Linux 5.4.57, we
> > > > twice saw a self-detected stall on a CPU (October 27th, 2020, January 18th,
> > > > 2021).
> > > > 
> > > > Both times, the workqueue is `xfs-conv/md0 xfs_end_io`.
> > > > 
> > > > ```
> > > > [    0.000000] Linux version 5.4.57.mx64.340
> > > > (root@theinternet.molgen.mpg.de) (gcc version 7.5.0 (GCC)) #1 SMP Tue Aug 11
> > > > 13:20:33 CEST 2020
> > > > […]
> > > > [48962.981257] rcu: INFO: rcu_sched self-detected stall on CPU
> > > > [48962.987511] rcu: 	4-....: (20999 ticks this GP)
> > > > idle=fe6/1/0x4000000000000002 softirq=3630188/3630188 fqs=4696
> > > > [48962.998805] 	(t=21017 jiffies g=14529009 q=32263)
> > > > [48963.004074] Task dump for CPU 4:
> > > > [48963.007689] kworker/4:2     R  running task        0 25587      2
> > > > 0x80004008
> > > > [48963.015591] Workqueue: xfs-conv/md0 xfs_end_io
> > > > [48963.020570] Call Trace:
> > > > [48963.023311]  <IRQ>
> > > > [48963.025560]  sched_show_task+0x11e/0x150
> > > > [48963.029957]  rcu_dump_cpu_stacks+0x70/0xa0
> > > > [48963.034545]  rcu_sched_clock_irq+0x502/0x770
> > > > [48963.039322]  ? tick_sched_do_timer+0x60/0x60
> > > > [48963.044106]  update_process_times+0x24/0x60
> > > > [48963.048791]  tick_sched_timer+0x37/0x70
> > > > [48963.053089]  __hrtimer_run_queues+0x11f/0x2b0
> > > > [48963.057960]  ? recalibrate_cpu_khz+0x10/0x10
> > > > [48963.062744]  hrtimer_interrupt+0xe5/0x240
> > > > [48963.067235]  smp_apic_timer_interrupt+0x6f/0x130
> > > > [48963.072407]  apic_timer_interrupt+0xf/0x20
> > > > [48963.076994]  </IRQ>
> > > > [48963.079347] RIP: 0010:_raw_spin_unlock_irqrestore+0xa/0x10
> > > > [48963.085491] Code: f3 90 83 e8 01 75 e8 65 8b 3d 42 0f 56 7e e8 ed ea 5e
> > > > ff 48 29 e8 4c 39 e8 76 cf 80 0b 08 eb 8c 0f 1f 44 00 00 c6 07 00 56 9d <c3>
> > > > 0f 1f 44 00 00 0f 1f 44 00 00 b8 00 fe ff ff f0 0f c1 07 56 9d
> > > > [48963.106524] RSP: 0018:ffffc9000738fd40 EFLAGS: 00000202 ORIG_RAX:
> > > > ffffffffffffff13
> > > > [48963.115003] RAX: ffffffff82407588 RBX: ffffffff82407580 RCX:
> > > > ffffffff82407588
> > > > [48963.122994] RDX: ffffffff82407588 RSI: 0000000000000202 RDI:
> > > > ffffffff82407580
> > > > [48963.130989] RBP: 0000000000000202 R08: ffffffff8203ea00 R09:
> > > > 0000000000000001
> > > > [48963.138982] R10: ffffc9000738fbb8 R11: 0000000000000001 R12:
> > > > ffffffff82407588
> > > > [48963.146976] R13: ffffea005e7ae600 R14: ffff8897b7e5a040 R15:
> > > > ffffea005e7ae600
> > > > [48963.154971]  wake_up_page_bit+0xe0/0x100
> > > > [48963.159366]  xfs_destroy_ioend+0xce/0x1c0
> > > > [48963.163857]  xfs_end_ioend+0xcf/0x1a0
> > > > [48963.167958]  xfs_end_io+0xa4/0xd0
> > > > [48963.171672]  process_one_work+0x1e5/0x410
> > > > [48963.176163]  worker_thread+0x2d/0x3c0
> > > > [48963.180265]  ? cancel_delayed_work+0x90/0x90
> > > > [48963.185048]  kthread+0x117/0x130
> > > > [48963.188663]  ? kthread_create_worker_on_cpu+0x70/0x70
> > > > [48963.194321]  ret_from_fork+0x35/0x40
> > > > ```
> > > > 
> > > > As it’s just log level INFO, is there anything what should be done, or was
> > > > the system probably just “overloaded”?
> > > 
> > > I am assuming that you are building your kernel with CONFIG_PREEMPT_NONE=y
> > > rather than CONFIG_PREEMPT_VOLUNTARY=y.
> > > 
> > > If so, and if the problem is that you are temporarily overdriving xfs I/O,
> > > one approach would be as follows:
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > > index f16d5f1..06be426 100644
> > > --- a/fs/xfs/xfs_aops.c
> > > +++ b/fs/xfs/xfs_aops.c
> > > @@ -390,6 +390,7 @@ xfs_end_io(
> > >  		list_del_init(&ioend->io_list);
> > >  		xfs_ioend_try_merge(ioend, &completion_list);
> > >  		xfs_end_ioend(ioend);
> > > +		cond_resched();
> > >  	}
> > >  }
> > >  
> > > ------------------------------------------------------------------------
> > 
> > FWIW, this looks quite similar to the problem I attempted to fix with
> > these patches:
> > 
> > https://lore.kernel.org/linux-xfs/20201002153357.56409-1-bfoster@redhat.com/
> 
> Looks plausible to me!  Do you plan to re-post taking the feedback
> into account?
> 

There was a v2 inline that incorporated some directed feedback.
Otherwise there were questions and ideas about making the whole thing
faster, but I've no idea if that addresses the problem or not (if so,
that would be an entirely different set of patches). I'll wait and see
what Darrick thinks about this and rebase/repost if the approach is
agreeable..

Brian

> 							Thanx, Paul
> 
> > Brian
> > 
> > > 
> > > If you have instead built with CONFIG_PREEMPT_VOLUNTARY=y, then your
> > > problem is likely massive lock contention in wake_up_page_bit(), or
> > > perhaps someone having failed to release that lock.  The usual way to
> > > work this out is by enabling lockdep (CONFIG_PROVE_LOCKING=y), but this
> > > is often not what you want enabled in production.
> > > 
> > > Darrick, thoughts from an xfs perspective?
> > > 
> > > 							Thanx, Paul
> > > 
> > 
>