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

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Date:	Mon, 14 Mar 2016 15:44:50 +0100
From:	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Anna-Maria Gleixner <anna-maria@...utronix.de>,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	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 Mon, Mar 14, 2016 at 09:51:50AM +0100, Heiko Carstens wrote:
> 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:

The change looks ok for me.

Acked-by: Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>

> >  		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.

That's right.  Allocation is done when an perf event is about to be scheduled
on a CPU.

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

That might be a problem for task-based perf events.  I will have a closer
look into that issue.

Many thanks.

Kind regards,
  Hendrik