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

[Waiman Long]

On 09/14/2015 10:10 AM, Peter Zijlstra wrote:
> 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?

The wait-early once logic was there because of the kick-ahead patch as I 
don't want a recently kicked vCPU near the head of the queue to go back 
to sleep too early. However, without kick-ahead, a woken up vCPU should 
now be at the queue head. Indeed, we can remove that check and simplify 
the logic.

BTW, the queue head vCPU at pv_wait_head_and_lock() doesn't wait early, 
it will spin the full threshold as there is no way for it to figure out 
if the lock holder is running or not.

Cheers,
Longman
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