Messages in this thread | | | Date | Thu, 5 Feb 2015 10:53:27 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v2] ARM: Don't use complete() during __cpu_die |
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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.kozlowski@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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