linux-kernel - Re: #tj-percpu has been rebased

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Date:	Wed, 18 Feb 2009 14:55:10 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>, cpw@....com
Subject: Re: #tj-percpu has been rebased

On Tuesday 17 February 2009 09:58:19 H. Peter Anvin wrote:
> Rusty Russell wrote:
> >>
> >> All in all I think a dedicated virtual zone per CPU as opposed to
> >> interleaving them seems to make more sense.  Even with 4096 CPUs and
> >> reserving, say, 256 MB per CPU it's not that much address space in the
> >> context of a 47-bit kernel space.  On 32 bits I don't think anything but
> >> the most trivial amount of percpu space is going to fly no matter what.
> > 
> > It's the TLB cost which I really don't want to pay; num_possible_cpus()
> > 4096 non-NUMA is a little silly (currently impossible).
> > 
> > I'm happy to limit per-cpu allocations to pagesize, then you only need to
> > find num_possible_cpus() contig pages, and if you can't, you fall back to
> > vmalloc.
> > 
> 
> num_possible_cpus() can be very large though, so in many cases the
> likelihood of finding that many pages approach zero.  Furthermore,
> num_possible_cpus() may be quite a bit larger than the actual number of
> CPUs in the system.

Sure, so we end up at vmalloc.  No worse, but simpler and much better if we
*can* do it.

Rusty.
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