linux-kernel - Re: [GIT PULL] slab fixes for 3.2-rc4

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Date:	Thu, 22 Dec 2011 19:03:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] slab fixes for 3.2-rc4


* Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 22 Dec 2011, Tejun Heo wrote:
> 
> > Yeap, and that one too.  Maybe we can finally kill the duplicate
> > confusing static/dynamic accessors too.  I'm planning to get to it in
> > several weeks but if anyone can beat me to it, please go ahead.
> 
> That would be great. I looked at _and and _or and they both still have one
> use case (_xor has none though). But its easy to get rid of the irqsafe
> variants once we are willing to take the additional overhead that comes
> with disabling interrupts for the fallback cases.
> 
> 
> Subject: [percpu] Remove irqsafe_cpu_xxx variants
> 
> We simply say that regular this_cpu use must be safe regardless of preemption
> and interrupt state. That has no material change for x86 and s390 implementations
> of this_cpu operations. However, arches that do not provide their own implementation
> for this_cpu operations will now get code generated that disables interrupts
> instead of preemption.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> ---
>  arch/s390/include/asm/percpu.h     |   50 ++++-----
>  arch/x86/include/asm/percpu.h      |   28 -----
>  include/linux/netdevice.h          |    4
>  include/linux/netfilter/x_tables.h |    4
>  include/linux/percpu.h             |  190 ++++---------------------------------
>  include/net/snmp.h                 |   14 +-
>  mm/slub.c                          |    6 -
>  net/caif/caif_dev.c                |    4
>  net/caif/cffrml.c                  |    4
>  9 files changed, 65 insertions(+), 239 deletions(-)

While this is progress, i think you have missed the essence of 
Linus's observations: percpu.h is *way* too complex and is 
offering way too many variants. The irqsafe madness was just the 
most blatant problem.

Note that even wit your patch applied linux/percpu.h is 800+ 
lines long, while the total number of usecases is smaller than 
that - and then i havent even considered all the arch percpu.h 
files.

Why not implement Linus's suggestion of just one or two 
__this_cpu() ops and be content with that model?

Thanks,

	Ingo
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