Re: [PATCH] ARM: Don't use complete() during __cpu_die

[Stephen Boyd]

On 02/04/15 08:53, 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:
>
> [   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@xxxxxxxxxxx>
> ---

Would it be better to use IPIs instead? The IPI handler could even call
complete() as long as we IPI off the dying CPU to the killing CPU. I
suppose this could be an extension of the current IPI_COMPLETION that we
already have.

>  arch/arm/kernel/smp.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 49 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 86ef244c5a24..bb8ff465975f 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -213,7 +219,40 @@ int __cpu_disable(void)
>  	return 0;
>  }
>  
> -static DECLARE_COMPLETION(cpu_died);
> +/*
> + * Wait for 5000*1 ms for 'wait_cpu_die' bit to be cleared.
> + * Actually the real wait time will be longer because of schedule()
> + * called bit_wait_timeout.
> + *
> + * Returns 0 if bit was cleared (CPU died) or non-zero
> + * otherwise (1 or negative ERRNO).
> + */
> +static int wait_for_cpu_die(void)
> +{
> +	int retries = 5000, sleep_ms = 1, ret = 0;
> +
> +	might_sleep();
> +
> +	smp_mb__before_atomic();
> +	while (test_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die)) {
> +		ret = out_of_line_wait_on_bit_timeout(&wait_cpu_die,
> +				CPU_DIE_WAIT_BIT, bit_wait_timeout,
> +				TASK_UNINTERRUPTIBLE,
> +				msecs_to_jiffies(sleep_ms));
> +		if (!ret || (--retries <= 0))
> +			break;
> +
> +		if (retries < 4000) {
> +			/* After ~1000 ms increase sleeping time to 10 ms */
> +			retries = 400;
> +			sleep_ms = 10;
> +		}
> +
> +		smp_mb__before_atomic(); /* For next test_bit() in loop */
> +	}
> +
> +	return ret;
> +}
>  

Is there any reason we test the bit before testing it again in
out_of_line_wait_on_bit_timeout()? Why can't we just call that function
in a loop and let it handle checking the bit?

> @@ -267,7 +312,8 @@ void __ref cpu_die(void)
>  	 * this returns, power and/or clocks can be removed at any point
>  	 * from this CPU and its cache by platform_cpu_kill().
>  	 */
> -	complete(&cpu_died);
> +	clear_bit(CPU_DIE_WAIT_BIT, &wait_cpu_die);
> +	smp_mb__after_atomic();
>  
>  	/*
>  	 * Ensure that the cache lines associated with that completion are

This comment here should be updated because the completion is gone.

-- 
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