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

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Date:	Wed, 10 Jun 2009 07:53:09 +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 10:08:53AM -0700, Linus Torvalds wrote:
> 
> 
> 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.

Sorry, I meant another task.

 
> > > 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).

The really theoretical one (which Andrew likes complaining about) is
when *everybody* is holding a kmap and asking for another one ;)
But I think it isn't too hard to make a pool for that. And yes we'd
also need a pool for atomic kmaps as you point out.
 
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