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SubjectRe: [PATCH 2/2] sched/hotplug: Ensure only per-cpu kthreads run during hotplug
On 2020-09-11 10:17:47 [+0200], Peter Zijlstra wrote:
> In preparation for migrate_disable(), make sure only per-cpu kthreads
> are allowed to run on !active CPUs.
>
> This is ran (as one of the very first steps) from the cpu-hotplug
> task which is a per-cpu kthread and completion of the hotplug
> operation only requires such tasks.
>
> This contraint enables the migrate_disable() implementation to wait
> for completion of all migrate_disable regions on this CPU at hotplug
> time without fear of any new ones starting.
>
> This replaces the unlikely(rq->balance_callbacks) test at the tail of
> context_switch with an unlikely(rq->balance_work), the fast path is
> not affected.

With this on top of -rc5 I get:

[ 42.678670] process 1816 (hackbench) no longer affine to cpu2
[ 42.678684] process 1817 (hackbench) no longer affine to cpu2
[ 42.710502] ------------[ cut here ]------------
[ 42.711505] rq->clock_update_flags < RQCF_ACT_SKIP
[ 42.711514] WARNING: CPU: 2 PID: 19 at kernel/sched/sched.h:1141 update_curr+0xe3/0x3f0
[ 42.714005] Modules linked in:
[ 42.714582] CPU: 2 PID: 19 Comm: migration/2 Not tainted 5.9.0-rc5+ #107
[ 42.715836] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1 04/01/2014
[ 42.717367] RIP: 0010:update_curr+0xe3/0x3f0
[ 42.718212] Code: 09 00 00 01 0f 87 6c ff ff ff 80 3d 52 bd 59 01 00 0f 85 5f ff ff ff 48 c7 c7 f8 7e 2b 82 c6 05 3e bd 59 01 01 e8 bc 9a fb ff <0f> 0b e9 45 ff ff ff 8b 05 d8 d4 59 01 48 8d 6b 80 8b 15 ba 5a 5b
[ 42.721839] RSP: 0018:ffffc900000e3d60 EFLAGS: 00010086
[ 42.722827] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[ 42.724166] RDX: ffff888275fa4540 RSI: ffffffff810f37e6 RDI: ffffffff82463940
[ 42.725547] RBP: ffff888276ca9600 R08: 0000000000000000 R09: 0000000000000026
[ 42.726925] R10: 0000000000000046 R11: 0000000000000046 R12: ffff888276ca9580
[ 42.728268] R13: ffff888276ca9580 R14: ffff888276ca9600 R15: ffff88826aa5c5c0
[ 42.729659] FS: 0000000000000000(0000) GS:ffff888276c80000(0000) knlGS:0000000000000000
[ 42.731186] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 42.732272] CR2: 00007f7952603f90 CR3: 000000026aa56000 CR4: 00000000003506e0
[ 42.733657] Call Trace:
[ 42.734136] dequeue_task_fair+0xfa/0x6f0
[ 42.734899] do_set_cpus_allowed+0xbb/0x1c0
[ 42.735692] cpuset_cpus_allowed_fallback+0x73/0x1a0
[ 42.736635] select_fallback_rq+0x129/0x160
[ 42.737450] __balance_push_stop+0x132/0x230
[ 42.738291] ? migration_cpu_stop+0x1f0/0x1f0
[ 42.739118] cpu_stopper_thread+0x76/0x100
[ 42.739897] ? smpboot_thread_fn+0x21/0x1f0
[ 42.740691] smpboot_thread_fn+0x106/0x1f0
[ 42.741487] ? __smpboot_create_thread.part.0+0x100/0x100
[ 42.742539] kthread+0x135/0x150
[ 42.743153] ? kthread_create_worker_on_cpu+0x60/0x60
[ 42.744106] ret_from_fork+0x22/0x30
[ 42.744792] irq event stamp: 100
[ 42.745433] hardirqs last enabled at (99): [<ffffffff81a8d46f>] _raw_spin_unlock_irq+0x1f/0x30
[ 42.747116] hardirqs last disabled at (100): [<ffffffff81a8d27e>] _raw_spin_lock_irq+0x3e/0x40
[ 42.748748] softirqs last enabled at (0): [<ffffffff81077b75>] copy_process+0x665/0x1b00
[ 42.750353] softirqs last disabled at (0): [<0000000000000000>] 0x0
[ 42.751552] ---[ end trace d98bb30ad2616d58 ]---

That comes due to DEQUEUE_NOCLOCK in do_set_cpus_allowed(). And then
there is this:

[ 42.752454] ======================================================
[ 42.752455] WARNING: possible circular locking dependency detected
[ 42.752455] 5.9.0-rc5+ #107 Not tainted
[ 42.752455] ------------------------------------------------------
[ 42.752456] migration/2/19 is trying to acquire lock:
[ 42.752456] ffffffff824639f8 ((console_sem).lock){-.-.}-{2:2}, at: down_trylock+0xa/0x30
[ 42.752458] but task is already holding lock:
[ 42.752458] ffff888276ca9598 (&rq->lock){-.-.}-{2:2}, at: __balance_push_stop+0x50/0x230
[ 42.752460] which lock already depends on the new lock.
[ 42.752460] the existing dependency chain (in reverse order) is:
[ 42.752461] -> #2 (&rq->lock){-.-.}-{2:2}:
[ 42.752462] _raw_spin_lock+0x27/0x40
[ 42.752462] task_fork_fair+0x30/0x1b0
[ 42.752462] sched_fork+0x10b/0x280
[ 42.752463] copy_process+0x702/0x1b00
[ 42.752464] _do_fork+0x5a/0x450
[ 42.752464] kernel_thread+0x50/0x70
[ 42.752465] rest_init+0x19/0x240
[ 42.752465] start_kernel+0x548/0x56a
[ 42.752466] secondary_startup_64+0xa4/0xb0

[ 42.752467] -> #1 (&p->pi_lock){-.-.}-{2:2}:
[ 42.752468] _raw_spin_lock_irqsave+0x36/0x50
[ 42.752469] try_to_wake_up+0x4e/0x720
[ 42.752469] up+0x3b/0x50
[ 42.752470] __up_console_sem+0x29/0x60
[ 42.752470] console_unlock+0x390/0x690
[ 42.752471] con_font_op+0x2ec/0x480
[ 42.752471] vt_ioctl+0x4d1/0x16e0
[ 42.752471] tty_ioctl+0x3e2/0x9a0
[ 42.752472] __x64_sys_ioctl+0x81/0x9a
[ 42.752472] do_syscall_64+0x33/0x40
[ 42.752472] entry_SYSCALL_64_after_hwframe+0x44/0xa9

[ 42.752473] -> #0 ((console_sem).lock){-.-.}-{2:2}:
[ 42.752474] __lock_acquire+0x11af/0x2010
[ 42.752474] lock_acquire+0xb7/0x400
[ 42.752474] _raw_spin_lock_irqsave+0x36/0x50
[ 42.752474] down_trylock+0xa/0x30
[ 42.752475] __down_trylock_console_sem+0x23/0x90
[ 42.752475] vprintk_emit+0xf6/0x380
[ 42.752476] printk+0x53/0x6a
[ 42.752476] __warn_printk+0x42/0x84
[ 42.752476] update_curr+0xe3/0x3f0
[ 42.752477] dequeue_task_fair+0xfa/0x6f0
[ 42.752477] do_set_cpus_allowed+0xbb/0x1c0
[ 42.752478] cpuset_cpus_allowed_fallback+0x73/0x1a0
[ 42.752479] select_fallback_rq+0x129/0x160
[ 42.752479] __balance_push_stop+0x132/0x230
[ 42.752480] cpu_stopper_thread+0x76/0x100
[ 42.752480] smpboot_thread_fn+0x106/0x1f0
[ 42.752480] kthread+0x135/0x150
[ 42.752480] ret_from_fork+0x22/0x30

[ 42.752481] other info that might help us debug this:
[ 42.752482] Chain exists of:
[ 42.752482] (console_sem).lock --> &p->pi_lock --> &rq->lock
[ 42.752483] Possible unsafe locking scenario:
[ 42.752484] CPU0 CPU1
[ 42.752484] ---- ----
[ 42.752484] lock(&rq->lock);
[ 42.752485] lock(&p->pi_lock);
[ 42.752486] lock(&rq->lock);
[ 42.752486] lock((console_sem).lock);
[ 42.752487] *** DEADLOCK ***
[ 42.752488] 3 locks held by migration/2/19:
[ 42.752488] #0: ffff88826aa5d0f8 (&p->pi_lock){-.-.}-{2:2}, at: __balance_push_stop+0x48/0x230
[ 42.752489] #1: ffff888276ca9598 (&rq->lock){-.-.}-{2:2}, at: __balance_push_stop+0x50/0x230
[ 42.752490] #2: ffffffff82478e00 (rcu_read_lock){....}-{1:2}, at: cpuset_cpus_allowed_fallback+0x0/0x1a0
[ 42.752491] stack backtrace:
[ 42.752492] CPU: 2 PID: 19 Comm: migration/2 Not tainted 5.9.0-rc5+ #107
[ 42.752492] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-1 04/01/2014
[ 42.752492] Call Trace:
[ 42.752493] dump_stack+0x77/0xa0
[ 42.752493] check_noncircular+0x156/0x170
[ 42.752493] __lock_acquire+0x11af/0x2010
[ 42.752493] ? __lock_acquire+0x1692/0x2010
[ 42.752493] lock_acquire+0xb7/0x400
[ 42.752494] ? down_trylock+0xa/0x30
[ 42.752494] ? log_store.constprop.0+0x1a4/0x250
[ 42.752494] ? printk+0x53/0x6a
[ 42.752494] _raw_spin_lock_irqsave+0x36/0x50
[ 42.752495] ? down_trylock+0xa/0x30
[ 42.752495] down_trylock+0xa/0x30
[ 42.752495] ? printk+0x53/0x6a
[ 42.752495] __down_trylock_console_sem+0x23/0x90
[ 42.752495] vprintk_emit+0xf6/0x380
[ 42.752496] printk+0x53/0x6a
[ 42.752496] __warn_printk+0x42/0x84
[ 42.752496] update_curr+0xe3/0x3f0
[ 42.752496] dequeue_task_fair+0xfa/0x6f0
[ 42.752497] do_set_cpus_allowed+0xbb/0x1c0
[ 42.752497] cpuset_cpus_allowed_fallback+0x73/0x1a0
[ 42.752497] select_fallback_rq+0x129/0x160
[ 42.752497] __balance_push_stop+0x132/0x230
[ 42.752497] ? migration_cpu_stop+0x1f0/0x1f0
[ 42.752498] cpu_stopper_thread+0x76/0x100
[ 42.752498] ? smpboot_thread_fn+0x21/0x1f0
[ 42.752498] smpboot_thread_fn+0x106/0x1f0
[ 42.752498] ? __smpboot_create_thread.part.0+0x100/0x100
[ 42.752499] kthread+0x135/0x150
[ 42.752499] ? kthread_create_worker_on_cpu+0x60/0x60
[ 42.752499] ret_from_fork+0x22/0x30
[ 42.878095] numa_remove_cpu cpu 2 node 0: mask now 0-1,3-7
[ 42.879977] smpboot: CPU 2 is now offline

Sebastian

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