On 04/22/2013 04:08 PM, Peter Zijlstra wrote:
On Mon, 2013-04-22 at 15:56 -0400, Rik van Riel wrote:
On 04/22/2013 03:49 PM, Peter Zijlstra wrote:
On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
If the native spin_lock code has been called already at
that time, the native code would still need to be modified
to increment the ticket number by 2, so we end up with a
compatible value in each spin lock's .tickets field, and
prevent a deadlock after we switch over to the paravirt
variant.
I thought the stuff already made it upstream, but apparently not; the
lastest posting I'm aware of is here:
https://lkml.org/lkml/2012/5/2/105
That stuff changes the normal ticket increment as well..
Jiannan,
It looks like the patch above could make a good patch
1 (or 2) in your patch series :)
I much prefer the entire series from Jeremy since it maintains the
ticket semantics and doesn't degrade the lock to unfair under
contention.
Now I suppose there's a reason its not been merged yet and I suspect
its !paravirt hotpath impact which wasn't rightly justified or somesuch
so maybe someone can work on that or so.. dunno.
IIRC one of the reasons was that the performance improvement wasn't
as obvious. Rescheduling VCPUs takes a fair amount of time, quite
probably more than the typical hold time of a spinlock.
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