Re: Preemptable Ticket Spinlock

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 04/22/2013 04:37 AM, Jiannan Ouyang wrote:
> On Sun, Apr 21, 2013 at 5:12 PM, Rik van Riel <riel@redhat.com> wrote:
>> Your algorithm is very clever, and very promising.
>>
>> However, it does increase the size of the struct spinlock, and adds
>> an additional atomic operation to spin_unlock, neither of which I
>> suspect are necessary.
>>
>> If we always incremented the ticket number by 2 (instead of 1), then
>> we could use the lower bit of the ticket number as the spinlock.
>>
>> If we do NOT run virtualized, we simply increment the ticket by 2
>> in spin_unlock, and the code can remain otherwise the same.
>>
>> If we do run virtualized, we take that spinlock after acquiring
>> the ticket (or timing out), just like in your code. In the
>> virtualized spin_unlock, we can then release the spinlock and
>> increment the ticket in one operation: by simply increasing the
>> ticket by 1.
>>
>> In other words, we should be able to keep the overhead of this
>> to an absolute minimum, and keep spin_unlock to be always the
>> same cost it is today.
>>
>> --
>> All rights reversed
>
> Hi Rik,
>
> Thanks for your feedback.
>
> Yes I agree with you
> - increase the size of struct spinlock is unnecessary
> - your idea of utilize the lower bit and save one atomic operation
> from unlock is cool!
>

Yes, +1. it is indeed a cool idea. Thanks to Jeremy.. and as Rik already 
mentioned it would also prevent the side effects of increasing
lock size. (It reminds my thought of encoding vcpuid in lock for pv 
spinlock)

> I can come up with a updated patch soon.
>
> --Jiannan
>
>