Linux Virtualization - Jul 2020 - [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR

On Thu, Jul 09, 2020 at 12:06:13PM -0400, Waiman Long
wrote:
> We don't really need to do a pv_spinlocks_init() if pv_kick() isn't
> supported.
Waiman, if you cannot explain how not having kick is a sane thing, what
are you saying here?

On 7/23/20 10:00 AM, Peter Zijlstra wrote:
> On Thu, Jul 09, 2020 at 12:06:13PM -0400, Waiman Long wrote:
>> We don't really need to do a pv_spinlocks_init() if pv_kick()
isn't
>> supported.
> Waiman, if you cannot explain how not having kick is a sane thing, what
> are you saying here?
>
The current PPC paravirt spinlock code doesn't do any cpu kick. It does 
an equivalence of pv_wait by yielding the cpu to the lock holder only. 
The pv_spinlocks_init() is for setting up the hash table for doing 
pv_kick. If we don't need to do pv_kick, we don't need the hash table.

I am not saying that pv_kick is not needed for the PPC environment. I 
was just trying to adapt the pvqspinlock code to such an environment 
first. Further investigation on how to implement some kind of pv_kick 
will be something that we may want to do as a follow on.

BTW, do you have any comment on my v2 lock holder cpu info qspinlock 
patch? I will have to update the patch to fix the reported 0-day test 
problem, but I want to collect other feedback before sending out v3.

Cheers,
Longman

On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long
wrote:
> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
> I will have to update the patch to fix the reported 0-day test problem, but
> I want to collect other feedback before sending out v3.
I want to say I hate it all, it adds instructions to a path we spend an
aweful lot of time optimizing without really getting anything back for
it.

Will, how do you feel about it?