linux-kernel - Re: [PATCH] Introduce x86_cpuinit.early_percpu_clock

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Date:	Tue, 7 Feb 2012 18:43:46 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Igor Mammedov <imammedo@...hat.com>
cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	jacob.jun.pan@...ux.intel.com, alan@...ux.intel.com,
	feng.tang@...el.com, konrad.wilk@...cle.com, avi@...hat.com,
	glommer@...hat.com, johnstul@...ibm.com, riel@...hat.com,
	tj@...nel.org, kosaki.motohiro@...fujitsu.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] Introduce x86_cpuinit.early_percpu_clock_init hook

On Tue, 7 Feb 2012, Igor Mammedov wrote:

> When kvm guest uses kvmclock, it may hang on vcpu hot-plug.
> This is caused by an overflow in pvclock_get_nsec_offset,
> 
>     u64 delta = tsc - shadow->tsc_timestamp;
> 
> which in turn is caused by an undefined values from percpu
> hv_clock that hasn't been initialized yet.
> Uninitialized clock on being booted cpu is accessed from
>    start_secondary
>     -> smp_callin
>       ->  smp_store_cpu_info
>         -> identify_secondary_cpu
>           -> mtrr_ap_init
>             -> mtrr_restore
>               -> stop_machine_from_inactive_cpu
>                 -> queue_stop_cpus_work
>                   ...
>                     -> sched_clock
>                       -> kvm_clock_read
> which is well before x86_cpuinit.setup_percpu_clockev call in
> start_secondary, where percpu clock is initialized.
> 
> This patch introduces a hook that allows to setup/initialize
> per_cpu clock early and avoid overflow due to reading
>   - undefined values
>   - old values if cpu was offlined and then onlined again
> 
> Another possible early user of this clock source is ftrace that
> accesses it to get timestamps for ring buffer entries. So if
> mtrr_ap_init is moved from identify_secondary_cpu to past
> x86_cpuinit.setup_percpu_clockev in start_secondary, ftrace
> may cause the same overflow/hang on cpu hot-plug anyway.
> 
> More complete description of the problem:
>   https://lkml.org/lkml/2012/2/2/101
> 
> Credits to Marcelo Tosatti <mtosatti@...hat.com> for hook idea.
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
>  arch/x86/include/asm/x86_init.h |    2 ++
>  arch/x86/kernel/kvmclock.c      |    4 +---
>  arch/x86/kernel/smpboot.c       |    1 +
>  arch/x86/kernel/x86_init.c      |    1 +
>  4 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
> index 517d476..5d0afac 100644
> --- a/arch/x86/include/asm/x86_init.h
> +++ b/arch/x86/include/asm/x86_init.h
> @@ -145,9 +145,11 @@ struct x86_init_ops {
>  /**
>   * struct x86_cpuinit_ops - platform specific cpu hotplug setups
>   * @setup_percpu_clockev:	set up the per cpu clock event device
> + * @early_percpu_clock_init:	early init of the per cpu clock event device

You initialize the per cpu clock, not the per cpu clock event
device. The latter is still initialized via setup_percpu_clockev().

Otherwise 

Acked-by: Thomas Gleixner <tglx@...utronix.de>

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