linux-kernel - Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

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Date:	Thu, 29 May 2008 22:49:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mike Travis <travis@....com>
Subject: Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

On Thu, 29 May 2008 22:27:53 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Thu, 29 May 2008, Andrew Morton wrote:
> 
> > > The per cpu memory use by subsystems is typically quite small. We already 
> > > have an 8k limitation for percpu space for modules. And that does not seem 
> > > to be a problem.
> > 
> > eh?  That's DEFINE_PERCPU memory, not alloc_pecpu() memory?
> 
> No. The module subsystem has its own alloc_percpu subsystem that the 
> cpu_alloc replaces.

That is to support DEFINE_PER_CPU, not alloc_percpu().

> > > We could do that yes.
> > 
> > Phew.
> 
> But its going to be even more complicated and I have a hard time managing 
> the complexity here. Could someone take pieces off my hand?

It could be done later on.

> > > But then per cpu data is not frequently allocated and freed.
> > 
> > I think it is, in the TCP case.  And that's the only one I looked at.
> 
> Which tcp case?

The one you just deleted from my reply :(

> > Plus who knows what lies ahead of us?
> 
> Well invariably we will end up with cpu area defragmentation.... Sigh.
> 
> > I don't think there is presently any upper limit on alloc_percpu()?  It
> > uses kmalloc() and kmalloc_node()?
> > 
> > Even if there is some limit, is it an unfixable one?
> 
> No there is no limit. It just wastes lots of space (pointer arrays, 
> alignment etc) that we could use to configure sufficiently large per cpu 
> areas.

Christoph, please.  An allocator which is of fixed size and which is
vulnerable to internal fragmentation is a huge problem!  The kernel is
subject to wildly varying workloads both between different users and in
the hands of a single user.

If we were to merge all this code and then run into the problems which
I fear then we are tremendously screwed.  We must examine this
exhaustively, in the most paranoid fashion.

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