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References: <20231103072639.11426-1-qiang.zhang1211@gmail.com>
In-Reply-To: <20231103072639.11426-1-qiang.zhang1211@gmail.com>
From:   Z qiang <qiang.zhang1211@gmail.com>
Date:   Fri, 3 Nov 2023 23:50:09 +0800
Message-ID: <CALm+0cVhrOL-CvQe41s6n75S-rk3mqO4RgS9y_wnRBr7YSm9CA@mail.gmail.com>
Subject: Re: [PATCH] rcutorture: Add fqs_holdoff check before the fqs_task is created
To:     paulmck@kernel.org, frederic@kernel.org, joel@joelfernandes.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

>
> For rcutorture tests that support fqs operations and set fqs_duration
> greater than zero, the fqs_task kthread will be created. but if the
> fqs_holdoff is not set, the default value is zero, this cause fqs_task
> enter a long-term busy loop and won't voluntarily give up the CPU until
> stopped by kthread_stop(). this commit therefore add for fqs_holdoff check
> before the fqs_task is created, make sure the fqs_task is created when
> the fqs_holdoff is also greater than zero.
>
> Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
>


if not apply this patch, will trigger rcu stall for built with
PREEMPT_RCU=n kernels.

runqemu kvm nographic slirp qemuparams="-smp 2 -m 1024"
bootparams="rcutorture.fqs_duration=4" -d

[   31.071252] rcu: INFO: rcu_sched self-detected stall on CPU
[   31.071264] rcu: 0-....: (25999 ticks this GP)
idle=a10c/1/0x4000000000000000 softirq=3064/3065 fqs=2038552
[   31.071273] rcu:          hardirqs   softirqs   csw/system
[   31.071277] rcu: number:        0        212            0
[   31.071281] rcu: cputime:      166          1        12830   ==> 12999(ms)
[   31.071286] rcu: (t=26000 jiffies g=1373 q=13803 ncpus=2)
[   31.071292] CPU: 0 PID: 48 Comm: rcu_torture_fqs Tainted: G
    L     6.6.0-rc2-rt4zqiang #54
[   31.071299] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
[   31.071303] RIP: 0010:_raw_spin_unlock_irqrestore+0x47/0x60
[   31.071314] Code: 08 e8 cd c6 92 fe 4c 89 e7 e8 b5 2d 93 fe 81 e3
00 02 00 00 75 1d 9c 58 f6 c4 02 75 1d 48 85 db 74 01 fb 65 ff 0d 51
86 98 66 <5b> b
[   31.071319] RSP: 0018:ffff888004237e38 EFLAGS: 00000246
[   31.071326] RAX: 0000000000000046 RBX: 0000000000000200 RCX: ffffffff981ab987
[   31.071331] RDX: 0000000000000003 RSI: dffffc0000000000 RDI: ffffffff99883445
[   31.071335] RBP: ffff888004237e48 R08: 0000000000000000 R09: 0000000000000000
[   31.071339] R10: fffffbfff3672d80 R11: ffffffff9b396c07 R12: ffffffff9ac02518
[   31.071344] R13: 0000000000000246 R14: 0000000000000000 R15: 0000000000000000
[   31.071348] FS:  0000000000000000(0000) GS:ffff888035e00000(0000)
knlGS:0000000000000000
[   31.071355] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   31.071359] CR2: 0000561df3203100 CR3: 0000000024a78000 CR4: 00000000001506f0
[   31.071364] Call Trace:
[   31.071367]  <IRQ>
[   31.071372]  ? show_regs+0x66/0x70
[   31.071379]  ? dump_cpu_task+0x68/0x70
[   31.071387]  ? rcu_dump_cpu_stacks+0x179/0x2b0
[   31.071396]  ? rcu_sched_clock_irq+0xb5c/0x11a0
[   31.071402]  ? irqtime_account_process_tick+0x158/0x210
[   31.071411]  ? update_process_times+0x6e/0xa0
[   31.071418]  ? tick_sched_timer+0x185/0x1d0
[   31.071425]  ? __pfx_tick_sched_timer+0x10/0x10
[   31.071431]  ? __hrtimer_run_queues+0x403/0x640
[   31.071438]  ? hrtimer_interrupt+0x77/0x350
[   31.071445]  ? __pfx___hrtimer_run_queues+0x10/0x10
[   31.071451]  ? kvm_clock_get_cycles+0x1c/0x30
[   31.071457]  ? ktime_get_update_offsets_now+0x102/0x200
[   31.071464]  ? hrtimer_interrupt+0x1ae/0x350
[   31.071473]  ? __sysvec_apic_timer_interrupt+0xaa/0x240
[   31.071480]  ? sysvec_apic_timer_interrupt+0x75/0x90
[   31.071487]  </IRQ>
[   31.071491]  <TASK>
[   31.071494]  ? asm_sysvec_apic_timer_interrupt+0x1f/0x30
[   31.071502]  ? lockdep_hardirqs_on_prepare+0x17/0x230
[   31.071510]  ? _raw_spin_unlock_irqrestore+0x55/0x60
[   31.071516]  ? _raw_spin_unlock_irqrestore+0x47/0x60
[   31.071524]  swake_up_one+0x47/0x60
[   31.071531]  rcu_gp_kthread_wake+0x63/0x90
[   31.071537]  rcu_force_quiescent_state+0xfb/0x160
[   31.071544]  rcu_torture_fqs+0xc4/0x160
[   31.071552]  ? __pfx_rcu_torture_fqs+0x10/0x10
[   31.071559]  kthread+0x197/0x1d0
[   31.071565]  ? kthread+0x107/0x1d0
[   31.071571]  ? __pfx_kthread+0x10/0x10
[   31.071577]  ret_from_fork+0x40/0x60
[   31.071583]  ? __pfx_kthread+0x10/0x10
[   31.071590]  ret_from_fork_asm+0x1b/0x30
[   31.071599]  </TASK>

Thanks
Zqiang

> ---
>  kernel/rcu/rcutorture.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 83999e57b3ea..9f0e6c1cad44 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -3886,7 +3886,9 @@ rcu_torture_init(void)
>         }
>         if (fqs_duration < 0)
>                 fqs_duration = 0;
> -       if (fqs_duration) {
> +       if (fqs_holdoff < 0)
> +               fqs_holdoff = 0;
> +       if (fqs_duration && fqs_holdoff) {
>                 /* Create the fqs thread */
>                 firsterr = torture_create_kthread(rcu_torture_fqs, NULL,
>                                                   fqs_task);
> --
> 2.17.1
>