linux-kernel - Re: [GIT PULL v2] Early SLAB fixes for 2.6.31

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Date:	Mon, 15 Jun 2009 18:11:01 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, kamezawa.hiroyu@...fujitsu.com,
	lizf@...fujitsu.com, mingo@...e.hu, npiggin@...e.de,
	yinghai@...nel.org, benh@...nel.crashing.org
Subject: Re: [GIT PULL v2] Early SLAB fixes for 2.6.31

Hi Christoph,

On Mon, 2009-06-15 at 11:05 -0400, Christoph Lameter wrote:
> On Mon, 15 Jun 2009, Pekka Enberg wrote:
> 
> > > Dont do it there. Only modify the slow path.
> > >
> > > Look at __might_sleep(). It already has an exception for system_state !=
> > > RUNNING. If it still triggers then add to the condition there.
> >
> > But does this matter? When the debugging options are turned off, there
> > are no users for "real_gfp" and thus GCC optimizes everything away. For
> > debugging configs, the extra cacheline load doesn't matter, does it?
> 
> It cleaner to have the fastpath as small as possible. Having unused
> variables in there is a bit confusing.

OK, I can clean this up, no problem.

On Mon, 2009-06-15 at 11:05 -0400, Christoph Lameter wrote:
> Also the path is performance critical. That may not matter for the debug
> case in some sitiations. But there are people that keep the debugging
> options on. Better to limit the impact as much as possible.
> 
> And the "extra cacheline load does not matter" reasoning can only be
> applied so many times. An extra cacheline load increases the cache foot
> print of the fast path after all.

The point here is that the actual debugging hooks do a whole lot more
than cacheline load + alu op even in the "fastpath" case (if such a
thing exists for those).

		Pekka

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