On Thu, Feb 05, 2015 at 10:14:30AM +0000, Krzysztof Kozlowski wrote:
> The complete() should not be used on offlined CPU. Rewrite the
> wait-complete mechanism with wait_on_bit_timeout().
>
> The CPU triggering hot unplug (e.g. CPU0) will loop until some bit is
> cleared. In each iteration schedule_timeout() is used with initial sleep
> time of 1 ms. Later it is increased to 10 ms.
>
> The dying CPU will clear the bit which is safe in that context.
>
> This fixes following RCU warning on ARMv8 (Exynos 4412, Trats2) during
> suspend to RAM:
Nit: isn't Exynos4412 a quad-A9 (ARMv7 rather than ARMv8)?
> [ 31.113925] ===============================
> [ 31.113928] [ INFO: suspicious RCU usage. ]
> [ 31.113935] 3.19.0-rc7-next-20150203 #1914 Not tainted
> [ 31.113938] -------------------------------
> [ 31.113943] kernel/sched/fair.c:4740 suspicious rcu_dereference_check()
> usage!
> [ 31.113946]
> [ 31.113946] other info that might help us debug this:
> [ 31.113946]
> [ 31.113952]
> [ 31.113952] RCU used illegally from offline CPU!
> [ 31.113952] rcu_scheduler_active = 1, debug_locks = 0
> [ 31.113957] 3 locks held by swapper/1/0:
> [ 31.113988] #0: ((cpu_died).wait.lock){......}, at: [<c005a114>]
> complete+0x14/0x44
> [ 31.114012] #1: (&p->pi_lock){-.-.-.}, at: [<c004a790>]
> try_to_wake_up+0x28/0x300
> [ 31.114035] #2: (rcu_read_lock){......}, at: [<c004f1b8>]
> select_task_rq_fair+0x5c/0xa04
> [ 31.114038]
> [ 31.114038] stack backtrace:
> [ 31.114046] CPU: 1 PID: 0 Comm: swapper/1 Not tainted
> 3.19.0-rc7-next-20150203 #1914
> [ 31.114050] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
> [ 31.114076] [<c0014ce4>] (unwind_backtrace) from [<c0011c30>]
> (show_stack+0x10/0x14)
> [ 31.114091] [<c0011c30>] (show_stack) from [<c04dc048>]
> (dump_stack+0x70/0xbc)
> [ 31.114105] [<c04dc048>] (dump_stack) from [<c004f83c>]
> (select_task_rq_fair+0x6e0/0xa04)
> [ 31.114118] [<c004f83c>] (select_task_rq_fair) from [<c004a83c>]
> (try_to_wake_up+0xd4/0x300)
> [ 31.114129] [<c004a83c>] (try_to_wake_up) from [<c00598a0>]
> (__wake_up_common+0x4c/0x80)
> [ 31.114140] [<c00598a0>] (__wake_up_common) from [<c00598e8>]
> (__wake_up_locked+0x14/0x1c)
> [ 31.114150] [<c00598e8>] (__wake_up_locked) from [<c005a134>]
> (complete+0x34/0x44)
> [ 31.114167] [<c005a134>] (complete) from [<c04d6ca4>] (cpu_die+0x24/0x84)
> [ 31.114179] [<c04d6ca4>] (cpu_die) from [<c005a508>]
> (cpu_startup_entry+0x328/0x358)
> [ 31.114189] [<c005a508>] (cpu_startup_entry) from [<40008784>] (0x40008784)
> [ 31.114226] CPU1: shutdown
>
> Signed-off-by: Krzysztof Kozlowski <k.kozlow...@samsung.com>
>
> ---
> Changes since v1:
> 1. Use adaptive sleep time when waiting for CPU die (idea and code
> from Paul E. McKenney). Paul also acked the patch but I made evem more
> changes.
>
> 2. Add another bit (CPU_DIE_TIMEOUT_BIT) for synchronizing power down
> failure in case:
> CPU0 (killing) CPUx (killed)
> wait_for_cpu_die
> timeout
> cpu_die()
> clear_bit()
> self power down
>
> In this case the bit would be cleared and CPU would be powered down
> introducing wrong behavior in next power down sequence (CPU0 would
> see the bit cleared).
> I think that such race is still possible but was narrowed to very
> short time frame. Any CPU up will reset the bit to proper values.
In the case of shutting down 2 CPUs in quick succession (without an
intervening boot of a CPU), surely this does not solve the potential
race on the wait_cpu_die variable?
I think we instead need a percpu synchronisation variable, which would
prevent racing on the value between CPUs, and a CPU would have to be
brought up before we could decide to kill it again. With that I think we
only need a single bit, too.
Thanks,
Mark.
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