linux-kernel - Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit

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Date:	Thu, 24 Mar 2011 08:52:35 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Nikanth Karthikesan" <knikanth@...e.de>
Cc:	"Nick Piggin" <npiggin@...nel.dk>, <x86@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...hat.com>, "Jack Steiner" <steiner@....com>,
	<linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86: avoid atomic operation in
	 test_and_set_bit_lock if possible

>>> On 24.03.11 at 05:56, Nikanth Karthikesan <knikanth@...e.de> wrote:
> On x86_64 SMP with lots of CPU atomic instructions which assert the LOCK #
> signal can stall other CPUs. And as the number of cores increase this 
> penalty
> scales proportionately. So it is best to try and avoid atomic instructions
> wherever possible. test_and_set_bit_lock() can avoid using LOCK_PREFIX if it
> finds the bit set already.

Why don't you do this in test_and_set_bit() instead?

Jan

> Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>
> 
> ---
> 
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index 903683b..26a42ff 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -203,19 +203,6 @@ static inline int test_and_set_bit(int nr, volatile 
> unsigned long *addr)
>  }
>  
>  /**
> - * test_and_set_bit_lock - Set a bit and return its old value for lock
> - * @nr: Bit to set
> - * @addr: Address to count from
> - *
> - * This is the same as test_and_set_bit on x86.
> - */
> -static __always_inline int
> -test_and_set_bit_lock(int nr, volatile unsigned long *addr)
> -{
> -	return test_and_set_bit(nr, addr);
> -}
> -
> -/**
>   * __test_and_set_bit - Set a bit and return its old value
>   * @nr: Bit to set
>   * @addr: Address to count from
> @@ -339,6 +326,25 @@ static int test_bit(int nr, const volatile unsigned long 
> *addr);
>  	 : variable_test_bit((nr), (addr)))
>  
>  /**
> + * test_and_set_bit_lock - Set a bit and return its old value for lock
> + * @nr: Bit to set
> + * @addr: Address to count from
> + *
> + * This is the same as test_and_set_bit on x86. But atomic operation is
> + * avoided, if the bit was already set.
> + */
> +static __always_inline int
> +test_and_set_bit_lock(int nr, volatile unsigned long *addr)
> +{
> +#ifdef CONFIG_SMP
> +	barrier();
> +	if (test_bit(nr, addr))
> +		return 1;
> +#endif
> +	return test_and_set_bit(nr, addr);
> +}
> +
> +/**
>   * __ffs - find first set bit in word
>   * @word: The word to search
>   *



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