Date: Fri, 4 Dec 2015 15:34:04 +0100
From: Borislav Petkov <bp@alien8.de>
To: Ashok Raj <ashok.raj@intel.com>
Cc: linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org,
        Tony Luck <tony.luck@intel.com>
Subject: Re: [Patch V0] x86, mce: Ensure offline CPU's don't participate in
 mce rendezvous process.
Message-ID: <20151204143404.GF21177@pd.tnic>
References: <1449188170-3909-1-git-send-email-ashok.raj@intel.com>
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On Thu, Dec 03, 2015 at 07:16:10PM -0500, Ashok Raj wrote:
> Linux has logical cpu offline capability. That can be triggered by:
> 
> # echo 0 > /sys/devices/system/cpu/cpuX/online
> 
> In Intel Architecture, MCE's are broadcasted to all CPUs in the system.
> 
> This includes the CPUs marked offline by Linux. Unless the CPU's were removed
> via an ACPI notification, in which case the cpu's are removed from the
> cpu_present_map.
> 
> This patch ensures offline CPU's don't participate in MCE rendezvous, but
> simply perform clearing some status bits to ensure a second MCE wont cause
> automatic shutdown.
> 
> Without the patch, mce_start will increment mce_callin, but mce_start would
> wait for all online_cpus. So offline cpu's should avoid participating in the
> rendezvous process.
> 
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> ---
>  arch/x86/kernel/cpu/mcheck/mce.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index c5b0d56..82a0c8b 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -998,6 +998,7 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>  	u64 recover_paddr = ~0ull;
>  	int flags = MF_ACTION_REQUIRED;
>  	int lmce = 0;
> +	unsigned int cpu = smp_processor_id();
>  
>  	ist_enter(regs);
>  
> @@ -1008,6 +1009,14 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>  
>  	mce_gather_info(&m, regs);
>  
> +	/*
> +	 * if this cpu is offline, just bail out.
> +	 * TBD: looking into adding any logs this offline CPU might have,
> +	 * to be collected and reported by the rendezvous master.
> +	 */
> +	if (cpu_is_offline(cpu) && (m.mcgstatus & MCG_STATUS_RIPV))
> +		goto out;

This CPU - it being offline and all - is not doing the minimal amount of
work possible IMO.

Why does it have to do ist_enter(), this_cpu_inc(mce_exception_count),
etc?

IMO the only things it should do is this:

	if (cpu_is_offline(smp_processor_id())) {
		mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
		return;
	}

and that should be at the very beginning of do_machine_check(). So
that the hardware is happy. Concerning Linux, it is offline so no data
structures on it are valid.

Hmmm?

P.S., please don't put stable@ to CC - add it as a "CC: " line in the
SOB section instead.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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