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

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Date:	Wed, 06 Jun 2012 11:58:38 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	mingo@...nel.org, 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

On 06/06/2012 10:24 AM, Peter Zijlstra 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).
> 

Actually, for this particular subcase I would use a synthetic CPUID bit
and use static_cpu_has().

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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