linux-kernel - Re: [tip:x86/mm] x86/pat: Avoid contention on cpa

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Date:	Wed, 6 Jun 2012 19:41:04 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, shai@...lemp.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, ido@...ery.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/pat: Avoid contention on cpa_lock if possible


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Wed, 2012-06-06 at 09:18 -0700, tip-bot for Shai Fultheim wrote:
> 
> > [ I absolutely hate these locking patterns ... yet I have no better idea. Maybe the gents on Cc: ... ]
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> 
> Oh yuck, this is vile..
> 
> static struct static_key scale_mp_trainwreck = STATIC_KEY_INIT_FALSE;
> 
> static DEFINE_SPINLOCK(_cpa_lock);
> 
> static inline void cpa_lock(void)
> {
> 	if (static_key_false(&scale_mp_trainwreck))
> 		return;
> 
> 	spin_lock(&_cpa_lock);
> }
> 
> static inline void cpa_unlock(void)
> {
> 	if (static_key_false(&scale_mp_trainwreck))
> 		return;
> 
> 	spin_lock(&_cpa_lock);
> }
> 
> And then use cpa_{,un}lock(), and the scale-mp guys can
> static_key_slow_inc(&scale_mp_trainwreck).
> 
> [ and yes I hate those jump_label names ... but I'm not wanting
>   to go through another round of bike-shed painting. ]

ok.

Another problem this patch has is inadequate testing:

arch/x86/mm/pageattr.c:798:2: error: implicit declaration of 
function ‘is_vsmp_box’ [-Werror=implicit-function-declaration]

So I'm removing it from tip:x86/mm for now.

Thanks,

	Ingo
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