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

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Message-ID: <alpine.LFD.2.01.0906090849080.6847@localhost.localdomain>
Date:	Tue, 9 Jun 2009 09:00:08 -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:
> 
> 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.

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.

If we can get rid of kmap_atomic(), I'd already like HIGHMEM more. Right 
now I absolutely _hate_ all the different "levels" of kmap_atomic() and 
having to be careful about crazy nesting rules etc.

		Linus
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