Re: [RFC] non-refcounted pages, application to slab?

[Eric Dumazet <dada1@cosmosbay.com>]

Nick Piggin a ecrit :

> @@ -2604,10 +2604,10 @@ static inline void *__cache_alloc(kmem_c
>  
>  	local_irq_save(save_flags);
>  	objp = ____cache_alloc(cachep, flags);
> +	prefetchw(objp);
>  	local_irq_restore(save_flags);
>  	objp = cache_alloc_debugcheck_after(cachep, flags, objp,
>  					    __builtin_return_address(0));
> -	prefetchw(objp);
>  	return objp;
>  }

I'm not sure why you moved this prefetchw(obj) : This is not related to your 
'non-refcounting' part, is it ?

When I added this prefetchw in slab code, I did place it *after* the 
local_irq_restore(save_flags); because I was not sure if the 
serialization/barrier (popf) would force the cpu (x86/x86_64 in mind) to either :
- finish all the loads (even if they are speculative/hints) (so giving a bad 
latency)
- cancel the speculative loads (so prefetchw() *before* the 
local_irq_restore() would be useless.

Thank you
Eric

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