linux-kernel - Re: [PATCH -v5 5/5] x86,smp: limit spinlock delay on virtual machines

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Date:	Thu, 7 Feb 2013 12:11:47 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com,
	eric.dumazet@...il.com, lwoodman@...hat.com, knoel@...hat.com,
	chegu_vinod@...com, raghavendra.kt@...ux.vnet.ibm.com,
	mingo@...hat.com
Subject: Re: [PATCH -v5 5/5] x86,smp: limit spinlock delay on virtual machines


* Rik van Riel <riel@...hat.com> wrote:

> --- a/arch/x86/kernel/cpu/hypervisor.c
> +++ b/arch/x86/kernel/cpu/hypervisor.c
> @@ -76,6 +76,8 @@ void __init init_hypervisor_platform(void)
>  
>  	init_hypervisor(&boot_cpu_data);
>  
> +	init_guest_spinlock_delay();
> +
>  	if (x86_hyper->init_platform)
>  		x86_hyper->init_platform();
>  }
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 64e33ef..fbc5ff3 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -116,8 +116,25 @@ static bool smp_no_nmi_ipi = false;
>  #define DELAY_SHIFT			8
>  #define DELAY_FIXED_1			(1<<DELAY_SHIFT)
>  #define MIN_SPINLOCK_DELAY		(1 * DELAY_FIXED_1)
> -#define MAX_SPINLOCK_DELAY		(16000 * DELAY_FIXED_1)
> +#define MAX_SPINLOCK_DELAY_NATIVE	(16000 * DELAY_FIXED_1)
> +#define MAX_SPINLOCK_DELAY_GUEST	(16 * DELAY_FIXED_1)
>  #define DELAY_HASH_SHIFT		6
> +
> +/*
> + * Modern Intel and AMD CPUs tell the hypervisor when a guest is
> + * spinning excessively on a spinlock. The hypervisor will then
> + * schedule something else, effectively taking care of the backoff
> + * for us. Doing our own backoff on top of the hypervisor's pause
> + * loop exit handling can lead to excessively long delays, and
> + * performance degradations. Limit the spinlock delay in virtual
> + * machines to a smaller value. Called from init_hypervisor_platform 
> + */
> +static int __read_mostly max_spinlock_delay = MAX_SPINLOCK_DELAY_NATIVE;
> +void __init init_guest_spinlock_delay(void)
> +{
> +	max_spinlock_delay = MAX_SPINLOCK_DELAY_GUEST;
> +}

The kernel build will be sad on !SMP configs.

Thanks,

	Ingo
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