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

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Date:	Wed, 18 Feb 2009 17:41:13 +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 Wednesday 18 February 2009 17:10:20 H. Peter Anvin wrote:
> Rusty Russell wrote:
> >>>
> >> 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.
> 
> If the likelihood is near zero, then you're wasting opportunities to do
> it better.  If we have compact per-cpu virtual areas then we can use
> large pages if we know we'll have large percpu areas.

You're right; we'd need that defrag wonderness people keep speculating about.

What finally convinced me is that the per-cpu chunks have to be at least the
size of the .data.percpu section (24k here).  7*num_possible_cpus() is even
worse.

Thanks,
Rusty.
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