linux-kernel - Re: [PATCH 08/11] qspinlock: Revert to test-and-set on hypervisors

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Date:	Wed, 18 Jun 2014 12:40:59 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Waiman.Long@...com, tglx@...utronix.de, mingo@...nel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	paolo.bonzini@...il.com, boris.ostrovsky@...cle.com,
	paulmck@...ux.vnet.ibm.com, riel@...hat.com,
	torvalds@...ux-foundation.org, raghavendra.kt@...ux.vnet.ibm.com,
	david.vrabel@...rix.com, oleg@...hat.com, gleb@...hat.com,
	scott.norton@...com, chegu_vinod@...com,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 08/11] qspinlock: Revert to test-and-set on hypervisors

On Sun, Jun 15, 2014 at 02:47:05PM +0200, Peter Zijlstra wrote:
> When we detect a hypervisor (!paravirt, see later patches), revert to

Please spell out the name of the patches.

> a simple test-and-set lock to avoid the horrors of queue preemption.

Heheh.
> 
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/x86/include/asm/qspinlock.h |   14 ++++++++++++++
>  include/asm-generic/qspinlock.h  |    7 +++++++
>  kernel/locking/qspinlock.c       |    3 +++
>  3 files changed, 24 insertions(+)
> 
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -1,6 +1,7 @@
>  #ifndef _ASM_X86_QSPINLOCK_H
>  #define _ASM_X86_QSPINLOCK_H
>  
> +#include <asm/cpufeature.h>
>  #include <asm-generic/qspinlock_types.h>
>  
>  #if !defined(CONFIG_X86_OOSTORE) && !defined(CONFIG_X86_PPRO_FENCE)
> @@ -20,6 +21,19 @@ static inline void queue_spin_unlock(str
>  
>  #endif /* !CONFIG_X86_OOSTORE && !CONFIG_X86_PPRO_FENCE */
>  
> +#define virt_queue_spin_lock virt_queue_spin_lock
> +
> +static inline bool virt_queue_spin_lock(struct qspinlock *lock)
> +{
> +	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return false;
> +
> +	while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
> +		cpu_relax();
> +
> +	return true;
> +}
> +
>  #include <asm-generic/qspinlock.h>
>  
>  #endif /* _ASM_X86_QSPINLOCK_H */
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -98,6 +98,13 @@ static __always_inline void queue_spin_u
>  }
>  #endif
>  
> +#ifndef virt_queue_spin_lock
> +static __always_inline bool virt_queue_spin_lock(struct qspinlock *lock)
> +{
> +	return false;
> +}
> +#endif
> +
>  /*
>   * Initializier
>   */
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -247,6 +247,9 @@ void queue_spin_lock_slowpath(struct qsp
>  
>  	BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
>  
> +	if (virt_queue_spin_lock(lock))
> +		return;
> +
>  	/*
>  	 * wait for in-progress pending->locked hand-overs
>  	 *
> 
> 
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