[PATCH v12 00/11] qspinlock: a 4-byte queue spinlock with PV support

Waiman Long
Mon, 27 Oct 2014 18:00:38 -0000

On 10/24/2014 04:57 AM, Peter Zijlstra wrote:
> On Thu, Oct 16, 2014 at 02:10:29PM -0400, Waiman Long wrote:
>> v11->v12:
>>   - Based on PeterZ's version of the qspinlock patch
>>     (https://lkml.org/lkml/2014/6/15/63).
>>   - Incorporated many of the review comments from Konrad Wilk and
>>     Paolo Bonzini.
>>   - The pvqspinlock code is largely from my previous version with
>>     PeterZ's way of going from queue tail to head and his idea of
>>     using callee saved calls to KVM and XEN codes.
> Thanks for taking the time to refresh this.. I would prefer you use a
> little more of the PV techniques I outlined in my latest PV patch to
> further reduce the overhead of PV enabled kernels on real hardware.
>
> This is an important use case, because distro kernels will have to
> enable PV support while their majority of installations will be on
> physical hardware.
>
> Other than that I see no reason not to move this forward.

Thanks for reviewing the patch and agree to move forward. Currently, I
am thinking of separating out a PV and non-PV versions of the lock
slowpath functions as shown in my previous mail. That should also
minimize the performance impact on bare metal even more than what can be
done with the PV techniques used in your patch while not penalizing PV
performance.

As for the unlock function, if the site patching function can handle all
the possible call sites of spin_unlock() without disabling function
inlining, I will be glad to use your way of handing unlock function.
Otherwise, I will prefer my current approach as it is simpler and more
easy to understand as well as similar to what has been done in the pv
ticket spinlock code.

-Longman