linux-kernel - Re: [PATCH 2/5] xen/spinlock: We don't need the old structure anymore

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Date:	Mon, 9 Sep 2013 16:06:46 +0530
From:	Ramkumar Ramachandra <artagnon@...il.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com, stefan.bader@...onical.com,
	stefano.stabellini@...citrix.com,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 2/5] xen/spinlock: We don't need the old structure anymore

Konrad Rzeszutek Wilk wrote:
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 0438b93..71db82c 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -81,7 +81,6 @@ static inline void spin_time_accum_blocked(u64 start)
>         spinlock_stats.time_blocked += delta;
>  }
>  #else  /* !CONFIG_XEN_DEBUG_FS */
> -#define TIMEOUT                        (1 << 10)

The timeout can be reduced, I think.

>  static inline void add_stats(enum xen_contention_stat var, u32 val)
>  {
>  }
> @@ -96,23 +95,6 @@ static inline void spin_time_accum_blocked(u64 start)
>  }
>  #endif  /* CONFIG_XEN_DEBUG_FS */
>
> -/*
> - * Size struct xen_spinlock so it's the same as arch_spinlock_t.
> - */
> -#if NR_CPUS < 256
> -typedef u8 xen_spinners_t;
> -# define inc_spinners(xl) \
> -       asm(LOCK_PREFIX " incb %0" : "+m" ((xl)->spinners) : : "memory");
> -# define dec_spinners(xl) \
> -       asm(LOCK_PREFIX " decb %0" : "+m" ((xl)->spinners) : : "memory");
> -#else
> -typedef u16 xen_spinners_t;
> -# define inc_spinners(xl) \
> -       asm(LOCK_PREFIX " incw %0" : "+m" ((xl)->spinners) : : "memory");
> -# define dec_spinners(xl) \
> -       asm(LOCK_PREFIX " decw %0" : "+m" ((xl)->spinners) : : "memory");
> -#endif
> -

Spinlocks on the filesystem help ensure consistency; otherwise, there
is a chance of a lot of noise coming through. Although NR_CPUS > 256,
very few CPUS are doing consistency checks.
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