Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications through the use of this cpu operations

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Christoph Lameter (cl@linux.com) wrote:
> We could do this with local cmpxchgs like in the following patch. This
> would avoid preemption disable and interrupt disable (at least on x86).
> Trouble is how do we make this fit for architectures that do not have
> cmpxchg?

All architectures should have a fallback nowadays, no ? This might involve
disabling interrupts around a cmpxchg emulation, which would make the slow path
disable/enable interrupts twice. Is it what you are concerned about ?

Thanks,

Matheu

> 
> 
> Index: linux-2.6/mm/vmstat.c
> ===================================================================
> --- linux-2.6.orig/mm/vmstat.c	2010-11-29 10:58:52.000000000 -0600
> +++ linux-2.6/mm/vmstat.c	2010-11-29 11:11:34.000000000 -0600
> @@ -169,18 +169,23 @@ void __mod_zone_page_state(struct zone *
>  {
>  	struct per_cpu_pageset __percpu *pcp = zone->pageset;
>  	s8 __percpu *p = pcp->vm_stat_diff + item;
> -	long x;
> -	long t;
> +	long o, n, t, z;
> 
> -	x = delta + __this_cpu_read(*p);
> +	do {
> +		z = 0;
> +		t = this_cpu_read(pcp->stat_threshold);
> +		o = this_cpu_read(*p);
> +		n = delta + o;
> +
> +		if (n > t || n < -t) {
> +			/* Overflow must be added to zone counters */
> +			z = n;
> +			n = 0;
> +		}
> +	} while (o != n && this_cpu_cmpxchg(*p, o, n) != o);
> 
> -	t = __this_cpu_read(pcp->stat_threshold);
> -
> -	if (unlikely(x > t || x < -t)) {
> -		zone_page_state_add(x, zone, item);
> -		x = 0;
> -	}
> -	__this_cpu_write(*p, x);
> +	if (z)
> +		zone_page_state_add(z, zone, item);
>  }
>  EXPORT_SYMBOL(__mod_zone_page_state);
> 
> @@ -190,11 +195,7 @@ EXPORT_SYMBOL(__mod_zone_page_state);
>  void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
>  					int delta)
>  {
> -	unsigned long flags;
> -
> -	local_irq_save(flags);
>  	__mod_zone_page_state(zone, item, delta);
> -	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL(mod_zone_page_state);
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com