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Message-ID: <7ed97096-859e-46d0-8f27-16a2298a8914@lucifer.local>
Date: Mon, 6 Jan 2025 10:52:37 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Koichiro Den <koichiro.den@...onical.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] vmstat: disable vmstat_work on vmstat_cpu_down_prep()
+cc tglx, peterz for insight on CPU hot plug
On Sat, Jan 04, 2025 at 01:00:17PM +0900, Koichiro Den wrote:
> On Fri, Jan 03, 2025 at 11:33:19PM +0000, Lorenzo Stoakes wrote:
> > On Sat, Dec 21, 2024 at 12:33:20PM +0900, Koichiro Den wrote:
> > > Even after mm/vmstat:online teardown, shepherd may still queue work for
> > > the dying cpu until the cpu is removed from online mask. While it's
> > > quite rare, this means that after unbind_workers() unbinds a per-cpu
> > > kworker, it potentially runs vmstat_update for the dying CPU on an
> > > irrelevant cpu before entering atomic AP states.
> > > When CONFIG_DEBUG_PREEMPT=y, it results in the following error with the
> > > backtrace.
> > >
> > > BUG: using smp_processor_id() in preemptible [00000000] code: \
> > > kworker/7:3/1702
> > > caller is refresh_cpu_vm_stats+0x235/0x5f0
> > > CPU: 0 UID: 0 PID: 1702 Comm: kworker/7:3 Tainted: G
> > > Tainted: [N]=TEST
> > > Workqueue: mm_percpu_wq vmstat_update
> > > Call Trace:
> > > <TASK>
> > > dump_stack_lvl+0x8d/0xb0
> > > check_preemption_disabled+0xce/0xe0
> > > refresh_cpu_vm_stats+0x235/0x5f0
> > > vmstat_update+0x17/0xa0
> > > process_one_work+0x869/0x1aa0
> > > worker_thread+0x5e5/0x1100
> > > kthread+0x29e/0x380
> > > ret_from_fork+0x2d/0x70
> > > ret_from_fork_asm+0x1a/0x30
> > > </TASK>
> > >
> > > So, for mm/vmstat:online, disable vmstat_work reliably on teardown and
> > > symmetrically enable it on startup.
> > >
> > > Signed-off-by: Koichiro Den <koichiro.den@...onical.com>
> >
> > Hi,
> >
> > I observed a warning in my qemu and real hardware, which I bisected to this commit:
> >
> > [ 0.087733] ------------[ cut here ]------------
> > [ 0.087733] workqueue: work disable count underflowed
> > [ 0.087733] WARNING: CPU: 1 PID: 21 at kernel/workqueue.c:4313 enable_work+0xb5/0xc0
> >
> > This is:
> >
> > static void work_offqd_enable(struct work_offq_data *offqd)
> > {
> > if (likely(offqd->disable > 0))
> > offqd->disable--;
> > else
> > WARN_ONCE(true, "workqueue: work disable count underflowed\n"); <-- this line
> > }
> >
> > So (based on this code) presumably an enable is only required if previously
> > disabled, and this code is being called on startup unconditionally without
> > the work having been disabled previously? I'm not hugely familiar with
> > delayed workqueue implementation details.
> >
> > [ 0.087733] Modules linked in:
> > [ 0.087733] CPU: 1 UID: 0 PID: 21 Comm: cpuhp/1 Not tainted 6.13.0-rc4+ #58
> > [ 0.087733] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
> > [ 0.087733] RIP: 0010:enable_work+0xb5/0xc0
> > [ 0.087733] Code: 6f b8 01 00 74 0f 31 d2 be 01 00 00 00 eb b5 90 0f 0b 90 eb ca c6 05 60 6f b8 01 01 90 48 c7 c7 b0 a9 6e 82 e8 4c a4 fd ff 90 <0f> 0b 90 90 eb d6 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90
> > [ 0.087733] RSP: 0018:ffffc900000cbe30 EFLAGS: 00010092
> > [ 0.087733] RAX: 0000000000000029 RBX: ffff888263ca9d60 RCX: 0000000000000000
> > [ 0.087733] RDX: 0000000000000001 RSI: ffffc900000cbce8 RDI: 0000000000000001
> > [ 0.087733] RBP: ffffc900000cbe30 R08: 00000000ffffdfff R09: ffffffff82b12f08
> > [ 0.087733] R10: 0000000000000003 R11: 0000000000000002 R12: 00000000000000c4
> > [ 0.087733] R13: ffffffff81278d90 R14: 0000000000000000 R15: ffff888263c9c648
> > [ 0.087733] FS: 0000000000000000(0000) GS:ffff888263c80000(0000) knlGS:0000000000000000
> > [ 0.087733] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 0.087733] CR2: 0000000000000000 CR3: 0000000002a2e000 CR4: 0000000000750ef0
> > [ 0.087733] PKRU: 55555554
> > [ 0.087733] Call Trace:
> > [ 0.087733] <TASK>
> > [ 0.087733] ? enable_work+0xb5/0xc0
> > [ 0.087733] ? __warn.cold+0x93/0xf2
> > [ 0.087733] ? enable_work+0xb5/0xc0
> > [ 0.087733] ? report_bug+0xff/0x140
> > [ 0.087733] ? handle_bug+0x54/0x90
> > [ 0.087733] ? exc_invalid_op+0x17/0x70
> > [ 0.087733] ? asm_exc_invalid_op+0x1a/0x20
> > [ 0.087733] ? __pfx_vmstat_cpu_online+0x10/0x10
> > [ 0.087733] ? enable_work+0xb5/0xc0
> > [ 0.087733] vmstat_cpu_online+0x5c/0x70
> > [ 0.087733] cpuhp_invoke_callback+0x133/0x440
> > [ 0.087733] cpuhp_thread_fun+0x95/0x150
> > [ 0.087733] smpboot_thread_fn+0xd5/0x1d0
> > [ 0.087734] ? __pfx_smpboot_thread_fn+0x10/0x10
> > [ 0.087735] kthread+0xc8/0xf0
> > [ 0.087737] ? __pfx_kthread+0x10/0x10
> > [ 0.087738] ret_from_fork+0x2c/0x50
> > [ 0.087739] ? __pfx_kthread+0x10/0x10
> > [ 0.087740] ret_from_fork_asm+0x1a/0x30
> > [ 0.087742] </TASK>
> > [ 0.087742] ---[ end trace 0000000000000000 ]---
> >
> >
> > > ---
> > > v1: https://lore.kernel.org/all/20241220134234.3809621-1-koichiro.den@canonical.com/
> > > ---
> > > mm/vmstat.c | 3 ++-
> > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > > index 4d016314a56c..0889b75cef14 100644
> > > --- a/mm/vmstat.c
> > > +++ b/mm/vmstat.c
> > > @@ -2148,13 +2148,14 @@ static int vmstat_cpu_online(unsigned int cpu)
> > > if (!node_state(cpu_to_node(cpu), N_CPU)) {
> > > node_set_state(cpu_to_node(cpu), N_CPU);
> > > }
> > > + enable_delayed_work(&per_cpu(vmstat_work, cpu));
> >
> > Probably needs to be 'if disabled' here, as this is invoked on normal
> > startup when the work won't have been disabled?
> >
> > Had a brief look at code and couldn't see how that could be done
> > however... and one would need to be careful about races... Tricky!
> >
> > >
> > > return 0;
> > > }
> > >
> > > static int vmstat_cpu_down_prep(unsigned int cpu)
> > > {
> > > - cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> > > + disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> > > return 0;
> > > }
> > >
> > > --
> > > 2.43.0
> > >
> > >
> >
> > Let me know if you need any more details, .config etc.
> >
> > I noticed this warning on a real box too (in both cases running akpm's
> > mm-unstable branch), FWIW.
>
> Thank you for the report. I was able to reproduce the warning and now
> wonder how I missed it.. My oversight, apologies.
>
> In my current view, the simplest solution would be to make sure a local
> vmstat_work is disabled until vmstat_cpu_online() runs for the cpu, even
> during boot-up. The following patch suppresses the warning:
>
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 0889b75cef14..19ceed5d34bf 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -2122,10 +2122,14 @@ static void __init start_shepherd_timer(void)
> {
> int cpu;
>
> - for_each_possible_cpu(cpu)
> + for_each_possible_cpu(cpu) {
> INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
> vmstat_update);
>
> + /* will be enabled on vmstat_cpu_online */
> + disable_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> + }
> +
> schedule_delayed_work(&shepherd,
> round_jiffies_relative(sysctl_stat_interval));
> }
>
> If you think of a better solution later, please let me know. Otherwise,
> I'll submit a follow-up fix patch with the above diff.
Thanks, this resolves the problem, but are we sure that _all_ CPUs will
definitely call vmstat_cpu_online()?
I did a bit of printk output and it seems like this _didn't_ online CPU 0,
presumably the boot CPU which calls this function in the first instance?
I also see that init_mm_internals() invokes cpuhp_setup_state_nocalls()
explicitly which does _not_ call the callback, though even if it did this
would be too early as it calls start_shepherd_timer() _after_ this anyway.
So yeah, unless I'm missing something, I think this patch is broken.
I have added Thomas and Peter to give some insight on the CPU hotplug side.
It feels like the patch really needs an 'enable if not already enabled'
call in vmstat_cpu_online().
>
> Thanks.
>
> -Koichiro
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