Re: [Xen-devel] [PATCH v16 08/14] pvqspinlock: Implement simple paravirt support for the qspinlock

[Waiman Long <waiman.long@xxxxxx>]

On 05/04/2015 10:20 AM, Peter Zijlstra wrote:
> I changed it to the below; I've not gotten around to compiling or even
> running it yet :-(
>
> The biggest change is the pv_hash/pv_unhash functions, which I've
> rewritten to hopefully be clearer (and also hopefully not wrecked them).
> I took out the cacheline sized structure which takes out that double
> loop and simplifies things. I've also added some comments which
> hopefully explain how/why we ended up with this exact scheme.
>
> I've also moved the __pv_queue_spin_unlock() function to the tail, such
> that we keep the 'wait'/'kick' order for both node and head.
>
> In any case, like I just wrote on the other email, I've stuck some
> things in my queue (up to and including patch 11) and if it all works
> out we can continue from there.
>
> ---
> Subject: pvqspinlock: Implement simple paravirt support for the qspinlock
> From: Waiman Long<Waiman.Long@xxxxxx>
> Date: Fri, 24 Apr 2015 14:56:37 -0400
>
> Provide a separate (second) version of the spin_lock_slowpath for
> paravirt along with a special unlock path.
>
> The second slowpath is generated by adding a few pv hooks to the
> normal slowpath, but where those will compile away for the native
> case, they expand into special wait/wake code for the pv version.
>
> The actual MCS queue can use extra storage in the mcs_nodes[] array to
> keep track of state and therefore uses directed wakeups.
>
> The head contender has no such storage directly visible to the
> unlocker.  So the unlocker searches a hash table with open addressing
> using a simple binary Galois linear feedback shift register.
I am fine with the change as it makes it simpler. BTW, I just saw a 
build error mail. That should be fixed easily with some minor edit.
Cheers,
Longman


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