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

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Message-ID: <Pine.LNX.4.64.0708141504350.32420@schroedinger.engr.sgi.com>
Date:	Tue, 14 Aug 2007 15:07:10 -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:

> > 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 */

No its not. Only on x64 which has implicit write ordering.

> 	spin_lock(&zone->lru_lock);
> 	...
> 	spin_unlock(&zone->lru_lock);
> 	__get_cpu(zone_flag) = 0;
> 	put_cpu();
> 
> Interrupt handler
> 
> 	if (!__get_cpu(zone_flag)) {

There are more spinlocks needed. So we would just check the whole bunch 
and fail if any of them are used?

> 		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) 

That would mean we need to fork the code for reclaim?
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