RE: [tip:x86/mm] x86/pat: Avoid contention on cpa_lock if possible

[Shai Fultheim (Shai@xxxxxxxxxxx)]

Peter Zijlstra [mailto:a.p.zijlstra@xxxxxxxxx] 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@xxxxxxxxxx>
> 
> 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. ]

Looks pretty straight forward to do.  
We will try this route, as I'm concerned that synthetic CPUID bit will be kind of a global change for a pretty local consideration.

Comments?

(and we will also fix the other error pointed by Ingo - we are missing an include in this patch)

Regards,
Shai.
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