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xen-devel
[Xen-devel] Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualiz
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To: |
"H. Peter Anvin" <hpa@xxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualization |
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From: |
Peter Zijlstra <peterz@xxxxxxxxxxxxx> |
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Date: |
Mon, 15 Nov 2010 21:14:48 +0100 |
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Cc: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, Srivatsa, Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Kivity <avi@xxxxxxxxxx>, Linux, Avi |
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Delivery-date: |
Mon, 15 Nov 2010 12:15:28 -0800 |
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<cover.1288794124.git.jeremy.fitzhardinge@xxxxxxxxxx> <4CDDBBD3.5050903@xxxxxxxxx> <4CDDBCE4.80906@xxxxxxxx> <4CDDBDB5.8000800@xxxxxxxxx> <4CE1915F.60507@xxxxxxxx> <4CE1920F.5000509@xxxxxxxxx> |
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On Mon, 2010-11-15 at 12:03 -0800, H. Peter Anvin wrote:
> On 11/15/2010 12:00 PM, Jeremy Fitzhardinge wrote:
> >
> > Another approach I discussed with PeterZ and Mathieu is to steal the LSB
> > of the ticket counters (halving the max CPU count) to use as a "there is
> > someone in slowpath waiting on this lock". But I haven't spent the time
> > to work out an algorithm to maintain that flag (or flags, since there
> > are bits available) in a correct and efficient way.
> >
>
> Definitely worth pondering.
Right, so the idea was to make the ticket increment 2, which would leave
the LSB of both the head and tail available. I think that if one were to
set both (using a cmpxchg), the ticket fast-path wouldn't need any
changes since head==tail is still the correct condition for acquisition.
Then the unlock needs an added conditional:
if (tail & 1)
unlock_slowpath()
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