linux-kernel - Re: [PATCH 00/13] [PATCH RFC] Paravirtualized ticketlocks

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Message-ID: <4E614DBD.3000504@goop.org>
Date:	Fri, 02 Sep 2011 14:42:21 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 00/13] [PATCH RFC] Paravirtualized ticketlocks

On 09/02/2011 01:27 PM, Linus Torvalds wrote:
> On Fri, Sep 2, 2011 at 1:07 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>> I don't know whether that fastpath code is small enough to consider
>> inlining everywhere?
> No.
>
> There's no point in inlining something that ends up containing a
> conditional function call: gcc will have to effectively save/restore
> registers around that thing anyway, so you lose a lot of the
> advantages of inlining. So I think it's better done as an out-of-line
> function, which I thought we did for spinlocks anyway.

Yes, lock currently out-of-line.

I should also make sure that unlock is also out of line when
paravirtualized.

> Also, do you run with CONFIG_OPTIMIZE_SIZE? Without that, gcc should
> be smart enough to make a "likely()" case be a fall-through.

Ah, I was wondering why I'd never seen likely/unlikely do anything
useful.  With OPTIMIZE_SIZE=n, there's no point in explicitly moving the
slowpath out to a separate function.

So the only downside with this variant is that it breaks my design
criteria of making the generated code look identical to the the original
code when CONFIG_PARAVIRT_SPINLOCKS=n.  But I don't know if that's an
actual downside in practice.

    J
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