linux-kernel - Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks

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Date:	Wed, 30 Jun 2010 11:11:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	jeremy.fitzhardinge@...rix.com, mingo@...e.hu, tglx@...utronix.de,
	Ky Srinivasan <KSrinivasan@...ell.com>,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks -
 base implementation

On Wed, 2010-06-30 at 10:00 +0100, Jan Beulich wrote:
> >>> On 30.06.10 at 10:05, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, 2010-06-29 at 15:31 +0100, Jan Beulich wrote:
> >> Add optional (alternative instructions based) callout hooks to the
> >> contended ticket lock and the ticket unlock paths, to allow hypervisor
> >> specific code to be used for reducing/eliminating the bad effects
> >> ticket locks have on performance when running virtualized.
> > 
> > Uhm, I'd much rather see a single alternative implementation, not a
> > per-hypervisor lock implementation.
> 
> How would you imaging this to work? I can't see how the mechanism
> could be hypervisor agnostic. Just look at the Xen implementation
> (patch 2) - do you really see room for meaningful abstraction there?

I tried not to, it made my eyes bleed.. 

But from what I hear all virt people are suffering from spinlocks (and
fair spinlocks in particular), so I was thinking it'd be a good idea to
get all interested parties to collaborate on one. Fragmentation like
this hardly ever works out well.

> Not the least that not every hypervisor may even have a way to
> poll for events (like Xen does), in which case a simple yield may be
> needed instead.

No idea what you're talking about, I think you assume I actually know
something about Xen or virt..

> >> For the moment, this isn't intended to be used together with pv-ops,
> >> but this is just to simplify initial integration. The ultimate goal
> >> for this should still be to replace pv-ops spinlocks.
> > 
> > So why not start by removing that?
> 
> Because I wouldn't get around to test it within the time constraints
> I have?

I'd say that removing basically dead code (the paravirt spinlocks) the
code you'd be changing was easier to follow and thus your patches would
be done quicker?

> >> +#define ALTERNATIVE_TICKET_LOCK \
> > 
> > But but but, the alternative isn't a ticket lock..!?
> 
> ??? Of course it is. 

Ah, right, after looking a bit more at patch 2 I see you indeed
implement a ticket like lock. Although why you need both a ticket and a
FIFO list is beyond me.

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