linux-kernel - Re: [RFC 4/9] Atomic reclaim: Save irq flags in vmscan.c

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Date:	Tue, 14 Aug 2007 23:56:59 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 4/9] Atomic reclaim: Save irq flags in vmscan.c

On Tue, Aug 14, 2007 at 02:48:31PM -0700, Christoph Lameter wrote:
> On Tue, 14 Aug 2007, Andi Kleen wrote:
> 
> > > But that still creates lots of overhead each time we take the lru lock!
> > 
> > A lot of overhead in what way? Setting a flag in a cache hot
> > per CPU data variable shouldn't be more than a few cycles.
> 
> Could you be a bit more specific? Where do you want to place the data?

DEFINE_PER_CPU(int, zone_flag);

	get_cpu();	// likely already true and then not needed
	__get_cpu(zone_flag) = 1;
	/* wmb is implied in spin_lock I think */
	spin_lock(&zone->lru_lock);
	...
	spin_unlock(&zone->lru_lock);
	__get_cpu(zone_flag) = 0;
	put_cpu();

Interrupt handler

	if (!__get_cpu(zone_flag)) {
		do things with zone locks 
	}

The interrupt handler shouldn't touch zone_flag. If it wants
to it would need to be converted to a local_t and incremented/decremented
(should be about the same cost at least on architectures with sane
local_t implementation) 
		
-Andi

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