linux-kernel - Re: [PATCH] arm64: kdump: Avoid to power off nonpanic CPUs

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Date:   Mon, 16 Oct 2017 09:08:46 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Hoeun Ryu <hoeun.ryu@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kdump: Avoid to power off nonpanic CPUs

Hi Mark,

On Tue, Oct 10, 2017 at 01:51:33PM -0600, Mathieu Poirier wrote:
> On 8 October 2017 at 09:35, Mark Rutland <mark.rutland@....com> wrote:
> > Hi Leo,
> >
> > On Sun, Oct 08, 2017 at 10:12:46PM +0800, Leo Yan wrote:
> >> commit a88ce63b642c ("arm64: kexec: have own crash_smp_send_stop() for
> >> crash dump for nonpanic cores") introduces ARM64 architecture function
> >> crash_smp_send_stop() to replace the weak function, this results in
> >> the nonpanic CPUs to be hot-plugged out and CPUs are placed into low
> >> power state on ARM64 platforms with the flow:
> >>
> >>   Panic CPU:
> >>     machine_crash_shutdown()
> >>       crash_smp_send_stop()
> >>       smp_cross_call(&mask, IPI_CPU_CRASH_STOP)
> >>
> >>   Nonpanic CPUs:
> >>     handle_IPI()
> >>       ipi_cpu_crash_stop()
> >>         cpu_ops[cpu]->cpu_die()
> >>
> >> The upper patch has no issue if enabled crash dump only; but if enabled
> >> crash dump and Coresight debug module for panic dumping at the meantime,
> >> nonpanic CPUs are powered off in crash dump flow,
> >
> > We want to turn secondary CPUs off if at all possible, since we want to prevent
> > issues resulting from asynchronous behaviour (e.g. TLB/cache fetches) that
> > could result in subsequent problems (e.g. if bad page tables resulted in page
> > table walks to MMIO devices).
> >
> > So we *really* want this behaviour in the general case.
> >
> >> later this may introduce conflicts with the Coresight debug module because
> >> Coresight debug registers dumping requires the CPU must be powered on for
> >> some platforms (e.g. Hi6220 on Hikey board). If we cannot keep the CPUs
> >> powered on, we can see the hardware lockup issue when access Coresight debug
> >> registers.
> >
> > Just to check I understand, the coresight debug module is being invoked as a
> > panic notifier in the current kernel, right?
> >
> >> To fix this issue, this commit removes CPU hotplug operation in func
> >> crash_smp_send_stop() and let CPUs to run into WFE/WFI states so CPUs
> >> can still be powered on after crash dump. This finally is more safe
> >> for Coresight debug module to dump registers and avoid hardware lockup.
> >>
> >> Cc: James Morse <james.morse@....com>
> >> Cc: Will Deacon <will.deacon@....com>
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> >> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> >> ---
> >>  arch/arm64/kernel/smp.c | 6 ------
> >>  1 file changed, 6 deletions(-)
> >>
> >> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> >> index 9f7195a..a65e68b 100644
> >> --- a/arch/arm64/kernel/smp.c
> >> +++ b/arch/arm64/kernel/smp.c
> >> @@ -856,12 +856,6 @@ static void ipi_cpu_crash_stop(unsigned int cpu, struct pt_regs *regs)
> >>
> >>       local_irq_disable();
> >>
> >> -#ifdef CONFIG_HOTPLUG_CPU
> >> -     if (cpu_ops[cpu]->cpu_die)
> >> -             cpu_ops[cpu]->cpu_die(cpu);
> >> -#endif
> >> -
> >
> > If it's really necessary to keep secondary CPUs online, please limit that to
> > the case where the coresight debug module is being used.
> >
> > IIRC there were similar interactions with cpuidle, and I don't see why hotplug
> > should be any different.
> 
> Can you point to where it was fixed for CPUidle?  We should try to do
> the same for coresight_debug so that things are done the same way.
> I'm also thinking that we could call ->cpu_die(cpu) in a #ifdef
> CONFIG_HOTPLUG_CPU clause in debug_notifier_call().  That way the
> behaviour remains the same, just enacted a little later - please
> advise on what option you prefer.

IMHO 's more readable to place hotplug operations into the function
ipi_cpu_crash_stop(), due this function is doing stuffs related with
"cpu stop".

But I think Mathieu's question is for you :) Could you give advice
as well?

Thanks,
Leo Yan