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

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Message-ID: <alpine.LFD.2.01.0906091005150.6847@localhost.localdomain>
Date:	Tue, 9 Jun 2009 10:08:53 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
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, 9 Jun 2009, Nick Piggin wrote:

> On Tue, Jun 09, 2009 at 09:26:47AM -0700, Linus Torvalds wrote:
> > 
> > 
> > On Tue, 9 Jun 2009, Nick Piggin wrote:
> > > 
> > > 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?).
> > 
> > No, everybody has to use kmap()/kunmap().
> 
> So it is strictly a bug to expose a pointer returned by kmap to
> another CPU?

No, not at all. The pointers are all global. They have to be, since the 
original kmap() user may well be scheduled away.

> > The "problem" is that you could in theory run out of kmap frames, since if 
> > everybody does a kmap() in an interruptible context and you have lots and 
> > lots of threads doing different pages, you'd run out. But that has nothing 
> > to do with kmap_atomic(), which is basically limited to just the number of 
> > CPU's and a (very small) level of nesting.
> 
> This could be avoided with an anti-deadlock pool. If a task
> attempts a nested kmap and already holds a kmap, then give it
> exclusive access to this pool until it releases its last
> nested kmap.

We just sleep, waiting for somebody to release their. Again, that 
obviously won't work in atomic context, but it's easy enough to just have 
a "we need to have a few entries free" for the atomic case, and make it 
busy-loop if it runs out (which is not going to happen in practice 
anyway).

			Linus
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