linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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Date:	Tue, 9 Jun 2009 18:21:25 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels

On Tue, Jun 09, 2009 at 09:00:08AM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 9 Jun 2009, Nick Piggin wrote:
> > 
> > If it's such a problem, it could be made a lot faster without too
> > much problem. You could just introduce a FIFO of ptes behind it 
> > and flush them all in one go. 4K worth of ptes per CPU might
> > hopefully bring your overhead down to < 1%.
> 
> We already have that. The regular kmap() does that. It's just not usable 
> in atomic context.

Well this would be more like the kmap cache idea rather than the
kmap_atomic FIFO (which would remain per-cpu and look much like
the existing kmap_atomic).


> We'd need to fix the locking: right now kmap_high() uses non-irq-safe 
> locks, and it does that whole cross-cpu flushing thing (which is why 
> those locks _have_ to be non-irq-safe.
> 
> The way to fix that, though, would be to never do any cross-cpu calls, and 
> instead just have a cpumask saying "you need to flush before you do 
> anything with kmap".  So you'd just set that cpumask inside the lock, and 
> if/when some other CPU does a kmap, they'd flush their local TLB at _that_ 
> point instead of having to have an IPI call.

The idea seems nice but isn't the problem that kmap gives back a
basically 1st class kernel virtual memory? (ie. it can then be used
by any other CPU at any point without it having to use kmap?).

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