linux-kernel - Re: [tip:perfcounters/core] x86: Add NMI types for kmap

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Date:	Wed, 17 Jun 2009 09:58:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	mingo@...hat.com, hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	efault@....de, npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic

On Tue, 2009-06-16 at 13:38 +0100, Hugh Dickins wrote:
> 
> Something else to throw in: what if they were not just atomic,
> but also replaced the current sleeping kmaps? i.e. a task context
> carries around its own stack of these.

I actually did that once, but it means the task needs to be cpu-affine,
because fixmaps have different addresses between cpus. And disabling
migration for tasks has subtle side-effects so I dropped that again.

However, I recently considered the possiblity of putting the fixmaps in
the new per-cpu address space so that we might use the %gs segment to
normalize the fixmap addresses between the cpus.

This would allow full preemptible kmaps (yay for -rt).

However I suspect it might greatly complicate kmaps for the !i386 world.
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