linux-kernel - Re: [PATCH 2/2] s390/cpum_sf: Fix cpu hotplug notifier transitions

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Date:	Mon, 14 Mar 2016 09:51:50 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Anna-Maria Gleixner <anna-maria@...utronix.de>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, rt@...utronix.de,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH 2/2] s390/cpum_sf: Fix cpu hotplug notifier transitions

On Fri, Mar 11, 2016 at 11:52:38AM +0100, Anna-Maria Gleixner wrote:
> The cpumf_pmu_notfier() hotplug callback lacks handling of the
> CPU_DOWN_FAILED case. That means, if CPU_DOWN_PREPARE failes, the PMC
> of the CPU is not setup again. Furthermore the CPU_ONLINE_FROZEN case
> will never be processed because of masking the switch expression with
> CPU_TASKS_FROZEN.
> 
> Add handling for CPU_DOWN_FAILED transition to setup the PMC of the
> CPU. Remove CPU_ONLINE_FROZEN case.
> 
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: linux-s390@...r.kernel.org
> Signed-off-by: Anna-Maria Gleixner <anna-maria@...utronix.de>
> ---
>  arch/s390/kernel/perf_cpum_sf.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/arch/s390/kernel/perf_cpum_sf.c
> +++ b/arch/s390/kernel/perf_cpum_sf.c
> @@ -1518,7 +1518,7 @@ static int cpumf_pmu_notifier(struct not
> 
>  	switch (action & ~CPU_TASKS_FROZEN) {
>  	case CPU_ONLINE:
> -	case CPU_ONLINE_FROZEN:
> +	case CPU_DOWN_FAILED:
>  		flags = PMC_INIT;
>  		smp_call_function_single(cpu, setup_pmc_cpu, &flags, 1);
>  		break;

This is a bit odd, but independent from your patch: setup_pmc_cpu() will
only deallocate buffers but not allocate them.

Looking at the code a bit further this seems to imply that sampling will
not work on cpus that were added later.

Hendrik, could you please have a look?