linux-kernel - Re: [PATCH -v4 5/5] x86,smp: limit spinlock delay on virtual machines

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Date:	Sat, 26 Jan 2013 13:00:43 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, aquini@...hat.com, walken@...gle.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 -v4 5/5] x86,smp: limit spinlock delay on virtual machines


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

>  static bool smp_no_nmi_ipi = false;
>  
>  /*
> + * 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.
> + */
> +#define DELAY_SHIFT 8
> +#define DELAY_FIXED_1 (1<<DELAY_SHIFT)
> +#define MIN_SPINLOCK_DELAY (1 * DELAY_FIXED_1)
> +#define MAX_SPINLOCK_DELAY_NATIVE (16000 * DELAY_FIXED_1)
> +#define MAX_SPINLOCK_DELAY_GUEST (16 * DELAY_FIXED_1)
> +static int __read_mostly max_spinlock_delay = MAX_SPINLOCK_DELAY_NATIVE;
> +void __init init_spinlock_delay(void)
> +{
> +	if (x86_hyper)
> +		max_spinlock_delay = MAX_SPINLOCK_DELAY_GUEST;

I realize that you took existing code and extended it, but that 
chunk of code looks pretty disgusting visually now - at minimum 
it should be vertically aligned as most other kernel code does.

The comment should also tell that the unit of these values is 
'spinlock-op loops' or so.

Thanks,

	Ingo
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