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

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Date:	Tue, 14 Aug 2007 14:48:31 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <andi@...stfloor.org>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 4/9] Atomic reclaim: Save irq flags in vmscan.c

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?

What we are talking about is

atomic_inc(&zone->reclaim_cpu[smp_processor_id()]);
smp_wmb();
spin_lock(&zone->lru_lock);

....

spin_unlock(&zone_lru_lock);
smp_wmb();
atomic_dec(&zone->reclaim_cpu[smp_processor_id()]);

That is not light weight.

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