Subject: Re: [PATCH v4] powerpc/pseries: Remove limit in wait for dying CPU

From: Nathan Lynch <[email-masked]>
Date: Tue, 30 Apr 2019 11:34:25 -0500

Thiago Jung Bauermann <[email-masked]> writes:
> This can be a problem because if the busy loop finishes too early, then the
> kernel may offline another CPU before the previous one finished dying,
> which would lead to two concurrent calls to rtas-stop-self, which is
> prohibited by the PAPR.
>
> Since the hotplug machinery already assumes that cpu_die() is going to
> work, we can simply loop until the CPU stops.
>
> Also change the loop to wait 100 µs between each call to
> smp_query_cpu_stopped() to avoid querying RTAS too often.

[...]

> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 97feb6e79f1a..d75cee60644c 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -214,13 +214,17 @@ static void pseries_cpu_die(unsigned int cpu)
>  			msleep(1);
>  		}
>  	} else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) {
> -
> -		for (tries = 0; tries < 25; tries++) {
> +		/*
> +		 * rtas_stop_self() panics if the CPU fails to stop and our
> +		 * callers already assume that we are going to succeed, so we
> +		 * can just loop until the CPU stops.
> +		 */
> +		while (true) {
>  			cpu_status = smp_query_cpu_stopped(pcpu);
>  			if (cpu_status == QCSS_STOPPED ||
>  			    cpu_status == QCSS_HARDWARE_ERROR)
>  				break;
> -			cpu_relax();
> +			udelay(100);
>  		}
>  	}

I agree with looping indefinitely but doesn't it need a cond_resched()
or similar check?