Re: [Xen-devel] [Patch 2 of 2]: PV-domain SMP performance Linux-part

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 16/01/2009 09:36, "Juergen Gross" <juergen.gross@xxxxxxxxxxxxxxxxxxx>
wrote:

>> The approach taken in Linux is not merely 'yield on spinlock' by the way, it
>> is 'block on event channel on spinlock' essentially turning a contended
>> spinlock into a sleeping mutex. I think that is quite different behaviour
>> from merely yielding, and expecting the scheduler to do something sensible
>> with your yield request.
> 
> Could you explain this a little bit more in detail, please?

Jeremy Fitzhardinge did the implementation for Linux, so I'm cc'ing him in
case he remembers more details than me.

Basically each CPU allocates itself an IPI event channel at start of day.
When a CPU attempts to acquire a spinlock it spins a short while (perhaps a
few microseconds?) and then adds itself to a bitmap stored in the lock
structure (I think, or it might be a linked list of sleepers?). It then
calls SCHEDOP_poll listing its IPI evtchn as its wakeup requirement. When
the lock holder releases the lock it checks for sleepers and if it sees one
then it pings one of them (or is it all of them?) on its event channel, thus
waking it to take the lock.

 -- Keir



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