linux-kernel - Re: [PATCH RFC 0/4] Paravirtual spinlocks

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Message-ID: <4872B6E2.5080003@goop.org>
Date:	Mon, 07 Jul 2008 17:37:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	virtualization@...ts.linux-foundation.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Xen devel <xen-devel@...ts.xensource.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@...ux-foundation.org>,
	Petr Tesarik <ptesarik@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Friebel <thomas.friebel@....com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH RFC 0/4] Paravirtual spinlocks

Rusty Russell wrote:
> On Tuesday 08 July 2008 05:07:49 Jeremy Fitzhardinge wrote:
>   
>> At the most recent Xen Summit, Thomas Friebel presented a paper
>> ("Preventing Guests from Spinning Around",
>> http://xen.org/files/xensummitboston08/LHP.pdf) investigating the
>> interactions between spinlocks and virtual machines.  Specifically, he
>> looked at what happens when a lock-holding VCPU gets involuntarily
>> preempted.
>>     
>
> I find it interesting that gang scheduling the guest was not suggested as an 
> obvious solution.
>   

It's an obvious answer, but not an obvious solution.  You trade off 
wasting time spinning vs wasting time waiting for N vcpus to be free for 
scheduling.  Or something; seems much more complex, particularly if you 
can do a small guest tweak to solve the problem.

> Anyway, concept looks fine; lguest's solution is more elegant of course :)
>   

You could remove all mutable state and call it "erlang".

> A little disappointing that you can't patch your version inline.

Spinlock code isn't inlined currently, so I hadn't considered it.  The 
fast path code for both lock and unlock is nearly small enough to 
consider it, but it seems a bit fiddly.

If the "spin_lock" and "spin_unlock" functions were inlined functions 
which called the out of line __raw_spin_lock/unlock functions, then 
after patching they would result in a direct call to the backend lock 
functions, which would be exactly equivalent to what happens now (since 
I hook __raw_spin_lock into calls via pv_lock_ops).

    J

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