Re: [PATCH v6 6/6] locking/pvqspinlock: Queue node adaptive spinning

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 11, 2015 at 02:37:38PM -0400, Waiman Long wrote:
> In an overcommitted guest where some vCPUs have to be halted to make
> forward progress in other areas, it is highly likely that a vCPU later
> in the spinlock queue will be spinning while the ones earlier in the
> queue would have been halted. The spinning in the later vCPUs is then
> just a waste of precious CPU cycles because they are not going to
> get the lock soon as the earlier ones have to be woken up and take
> their turn to get the lock.
> 
> This patch implements an adaptive spinning mechanism where the vCPU
> will call pv_wait() if the following conditions are true:
> 
>  1) the vCPU has not been halted before;
>  2) the previous vCPU is not running.

Why 1? For the mutex adaptive stuff we only care about the lock holder
running, right?