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Message-Id: <20211011172236.11223-1-mkoutny@suse.com>
Date: Mon, 11 Oct 2021 19:22:36 +0200
From: Michal Koutný <mkoutny@...e.com>
To: linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Odin Ugedal <odin@...d.al>
Subject: [PATCH] sched/fair: Use rq->lock when checking cfs_rq list presence
The removal path checks cfs_rq->on_list flag without synchronization
based on the reasoning that only empty cgroups can be removed and
->on_list can't switch back to 1. However, since the commit a7b359fc6a37
("sched/fair: Correctly insert cfs_rq's to list on unthrottle") even
cfs_rq of an empty cgroup may be added to the list because of
non-decayed load that transiently remains there.
The result is that we may skip unlinking the cfs_rq from the list on the
removal path and the list will then contain free'd cfs_rq, which leads
sooner or later to a task failing inside update_blocked_averages while
holding rq->lock and eventually locking up the machine on all other CPUs
as well:
[ 8995.095798] BUG: kernel NULL pointer dereference, address: 0000000000000080
[ 9016.281685] NMI watchdog: Watchdog detected hard LOCKUP on cpu 21
Illustrative stack dump of a task that faulted by accessing released
cfs_rq (+unrelated deadlock on rq->lock):
PID: 0 TASK: ffff8a310a5dc000 CPU: 16 COMMAND: "swapper/16"
#0 [fffffe0000379e58] crash_nmi_callback at ffffffffba063683
#1 [fffffe0000379e60] nmi_handle at ffffffffba0377ef
#2 [fffffe0000379eb8] default_do_nmi at ffffffffba037c5e
#3 [fffffe0000379ed8] do_nmi at ffffffffba037ea7
#4 [fffffe0000379ef0] end_repeat_nmi at ffffffffbaa0178b
[exception RIP: native_queued_spin_lock_slowpath+98]
RIP: ffffffffba0fa6e2 RSP: ffffa8505932ca30 RFLAGS: 00000002
RAX: 0000000001200101 RBX: ffff8a8de9044000 RCX: ffff8a8ec0800000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8a8ec082cc80
RBP: ffff8a8ec082cc80 R8: ffff8a8ec0800000 R9: ffff8a31078058f8
R10: 0000000000000000 R11: ffffffffbb4639d8 R12: 0000000000000000
R13: ffff8a8de9044b84 R14: 0000000000000006 R15: 0000000000000010
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
--- <NMI exception stack> ---
#5 [ffffa8505932ca30] native_queued_spin_lock_slowpath at ffffffffba0fa6e2
#6 [ffffa8505932ca30] _raw_spin_lock at ffffffffba922aab
#7 [ffffa8505932ca38] try_to_wake_up at ffffffffba0cf8f9
#8 [ffffa8505932ca98] __queue_work at ffffffffba0b9c7e
#9 [ffffa8505932cae0] queue_work_on at ffffffffba0ba7b4
#10 [ffffa8505932caf0] bit_putcs at ffffffffba541bc0
#11 [ffffa8505932cbf0] fbcon_putcs at ffffffffba53c36b
#12 [ffffa8505932cc48] vt_console_print at ffffffffba5ff032
#13 [ffffa8505932cca8] console_unlock at ffffffffba1091a2
#14 [ffffa8505932ccf0] vprintk_emit at ffffffffba10ad29
#15 [ffffa8505932cd40] printk at ffffffffba10b590
#16 [ffffa8505932cda8] no_context at ffffffffba081f66
#17 [ffffa8505932ce10] do_page_fault at ffffffffba082df0
#18 [ffffa8505932ce40] page_fault at ffffffffbaa012fe
[exception RIP: update_blocked_averages+685]
RIP: ffffffffba0d85cd RSP: ffffa8505932cef0 RFLAGS: 00010046
RAX: 0000000000000000 RBX: ffff8a8ca0510000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff8a8ca0510000 RDI: 0000000000000000
RBP: 0000000000000000 R8: 00000000eac0c6e6 R9: 0000000000000233
R10: ffffa8505932cef0 R11: 0000000000000233 R12: ffff8a8ca0510140
R13: 0000000000000000 R14: fffffffffffffec2 R15: 0000000000000080
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
#19 [ffffa8505932cf50] run_rebalance_domains at ffffffffba0e2751
#20 [ffffa8505932cf68] __softirqentry_text_start at ffffffffbac000e3
#21 [ffffa8505932cfc8] irq_exit at ffffffffba0a2cf5
#22 [ffffa8505932cfd8] smp_apic_timer_interrupt at ffffffffbaa02874
#23 [ffffa8505932cff0] apic_timer_interrupt at ffffffffbaa01d9f
--- <IRQ stack> ---
#24 [ffffa850402f3dd8] apic_timer_interrupt at ffffffffbaa01d9f
[exception RIP: cpuidle_enter_state+171]
RIP: ffffffffba6fc32b RSP: ffffa850402f3e80 RFLAGS: 00000246
RAX: ffff8a8ec082cc80 RBX: ffffc7f053605e80 RCX: 000000000000001f
RDX: 0000082e557d390c RSI: 000000003d1877c2 RDI: 0000000000000000
RBP: ffffffffbb55f100 R8: 0000000000000002 R9: 000000000002c500
R10: ffffa850402f3e60 R11: 00000000000002ff R12: 0000000000000002
R13: 0000082e557d390c R14: 0000000000000002 R15: 0000000000000000
ORIG_RAX: ffffffffffffff13 CS: 0010 SS: 0018
#25 [ffffa850402f3ec0] cpuidle_enter at ffffffffba6fc6f9
#26 [ffffa850402f3ee0] do_idle at ffffffffba0d4567
#27 [ffffa850402f3f20] cpu_startup_entry at ffffffffba0d4769
#28 [ffffa850402f3f30] start_secondary at ffffffffba064e35
#29 [ffffa850402f3f50] secondary_startup_64_no_verify at ffffffffba000112
Fix this by always taking rq->lock when checking the ->on_list condition
(the modification of the list in UBA is therefore synchronized).
Taking the rq->lock on every cpu cgroup removal may pose a performance
penalty. However, this should be just moving the necessary work from UBA
into the unregister_fair_sched_group() and therefore neutral on larger
scale (assuming given cpu cgroup was populated at least once).
Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Signed-off-by: Michal Koutný <mkoutny@...e.com>
---
kernel/sched/fair.c | 7 -------
1 file changed, 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6a05d9b5443..081c7ac02f65 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11440,13 +11440,6 @@ void unregister_fair_sched_group(struct task_group *tg)
if (tg->se[cpu])
remove_entity_load_avg(tg->se[cpu]);
- /*
- * Only empty task groups can be destroyed; so we can speculatively
- * check on_list without danger of it being re-added.
- */
- if (!tg->cfs_rq[cpu]->on_list)
- continue;
-
rq = cpu_rq(cpu);
raw_spin_rq_lock_irqsave(rq, flags);
--
2.33.0
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