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Date:   Tue, 12 Jan 2021 09:14:11 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively

On Mon, Jan 11, 2021 at 01:50:52PM -0800, Paul E. McKenney wrote:
> On Mon, Jan 11, 2021 at 10:09:07AM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 11, 2021 at 06:16:39PM +0100, Peter Zijlstra wrote:
> > > 
> > > While thinking more about this, I'm thinking a big part of the problem
> > > is that we're not dinstinguishing between geniuine per-cpu kthreads and
> > > kthreads that just happen to be per-cpu.
> > > 
> > > Geniuine per-cpu kthreads are kthread_bind() and have PF_NO_SETAFFINITY,
> > > but sadly a lot of non-per-cpu kthreads, that might happen to still be
> > > per-cpu also have that -- again workqueue does that even to it's unbound
> > > workers :-(
> > > 
> > > Now, anything created by smpboot, is created through
> > > kthread_create_on_cpu() and that additionally sets to_kthread(p)->flags
> > > KTHREAD_IS_PER_CPU.
> > > 
> > > And I'm thinking that might be sufficient, if we modify
> > > is_per_cpu_kthread() to check that, then we only match smpboot threads
> > > (which include the hotplug and stopper threads, but notably not the idle
> > > thread)
> > > 
> > > Sadly it appears like io_uring() uses kthread_create_on_cpu() without
> > > then having any hotplug crud on, so that needs additinoal frobbing.
> > > 
> > > Also, init_task is PF_KTHREAD but doesn't have a struct kthread on.. and
> > > I suppose bound workqueues don't go through this either.
> > > 
> > > Let me rummage around a bit...
> > > 
> > > This seems to not insta-explode... opinions?
> > 
> > It passes quick tests on -rcu both with and without the rcutorture fixes,
> > which is encouraging.  I will start a more vigorous test in about an hour.
> 
> And 672 ten-minute instances of RUDE01 passed with this patch applied
> and with my rcutorture patch reverted.  So looking good, thank you!!!

Still on the yesterday's patch, an overnight 12-hour run hit workqueue
warnings in three of four instances of the SRCU-P scenario, two
at not quite three hours in and the third at about ten hours in.
All runs were otherwise successful.  One of the runs also had "BUG:
using __this_cpu_read() in preemptible" as well, so that is the warning
shown below.  There was a series of these BUGs, then things settled down.

This is the warning at the end of process_one_work() that is complaining
about being on the wrong CPU.

I will fire up some tests on the new series.

							Thanx, Paul

------------------------------------------------------------------------

WARNING: CPU: 0 PID: 413 at kernel/workqueue.c:2193 process_one_work+0x8c/0x5f0
Modules linked in:
CPU: 0 PID: 413 Comm: kworker/3:3 Not tainted 5.11.0-rc3+ #1104
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.11.0-2.el7 04/01/2014
Workqueue:  0x0 (events)
RIP: 0010:process_one_work+0x8c/0x5f0
Code: 48 8b 46 38 41 83 e6 20 48 89 45 c0 48 8b 46 40 48 89 45 c8 41 f6 44 24 4c 04 75 10 65 8b 05 eb 5d 78 59 41 39 44 24 40 74 02 <0f> 0b 48 ba eb 83 b5 80 46 86 c8 61 48 0f af d3 48 c1 ea 3a 49 8b
RSP: 0018:ffffb5a540847e70 EFLAGS: 00010006
RAX: 0000000000000000 RBX: ffff8fcc5f4f27e0 RCX: 2b970af959bb2a7d
RDX: ffff8fcc5f4f27e8 RSI: ffff8fcc5f4f27e0 RDI: ffff8fcc4306e3c0
RBP: ffffb5a540847ed0 R08: 0000000000000001 R09: ffff8fcc425e4680
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8fcc5f4eadc0
R13: ffff8fcc5f4ef700 R14: 0000000000000000 R15: ffff8fcc4306e3c0
FS:  0000000000000000(0000) GS:ffff8fcc5f400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004001e1 CR3: 0000000003084000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ? process_one_work+0x5f0/0x5f0
 worker_thread+0x28/0x3c0
 ? process_one_work+0x5f0/0x5f0
 kthread+0x13b/0x160
 ? kthread_insert_work_sanity_check+0x50/0x50
 ret_from_fork+0x22/0x30
irq event stamp: 138141554
hardirqs last  enabled at (138141553): [<ffffffffa74a928f>] _raw_spin_unlock_irq+0x1f/0x40
hardirqs last disabled at (138141554): [<ffffffffa74a9071>] _raw_spin_lock_irq+0x41/0x50
softirqs last  enabled at (138140828): [<ffffffffa68ece37>] srcu_invoke_callbacks+0xe7/0x1a0
softirqs last disabled at (138140824): [<ffffffffa68ece37>] srcu_invoke_callbacks+0xe7/0x1a0
---[ end trace e31d6dded2c52564 ]---
kvm-guest: stealtime: cpu 3, msr 1f4d7b00
BUG: using __this_cpu_read() in preemptible [00000000] code: kworker/3:3/413
caller is refresh_cpu_vm_stats+0x1a6/0x320
CPU: 5 PID: 413 Comm: kworker/3:3 Tainted: G        W         5.11.0-rc3+ #1104
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.11.0-2.el7 04/01/2014
Workqueue: mm_percpu_wq vmstat_update
Call Trace:
 dump_stack+0x77/0x97
 check_preemption_disabled+0xb6/0xd0
 refresh_cpu_vm_stats+0x1a6/0x320
 vmstat_update+0xe/0x60
 process_one_work+0x2a0/0x5f0
 ? process_one_work+0x5f0/0x5f0
 worker_thread+0x28/0x3c0
 ? process_one_work+0x5f0/0x5f0
 kthread+0x13b/0x160
 ? kthread_insert_work_sanity_check+0x50/0x50
 ret_from_fork+0x22/0x30

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