Re: [PATCH v3 0/9] s390: Improve this_cpu operations

[Yang Shi <yang@os.amperecomputing.com>]

On 5/27/26 12:09 PM, Christoph Lameter (Ampere) wrote:
> On Fri, 22 May 2026, Heiko Carstens wrote:
>
>> Also with the current proposal I only did some quick micro benchmarks,
>> which resulted in 0-1% improvement, which is in the expected range.
>>
>> It is amazing to see the performance improvements you see on arm64, however
>> I believe that is mainly because of the large amount of code which is
>> generated by the arm64 implementations of the preempt primitives
>> __preempt_count_add() and __preempt_count_dec_and_test().
> The code is generated if you have no arch specific per cpu mechanism and
> preemption must be supported. We have now the situation that we cannot
> switch off preemption support anymore.
>
> It seem that S390 has this mechanism in a small way and therefore can
> avoid the preempt enable/disable.
>
> It is not the quantity of code here. The preempt enable/disable can only
> be avoided if there is a single instruction doing the per cpu operation. A
> single instruction cannot be interupted and therefore is preemption safe.
>
>
>> That's a big difference to s390: for both primitives the result is a single
>> instruction.
> Ok then you can already use single instructions like x86 and will not have
> preempt enable/disable overhead.

I don't think S390 can do it in one single instruction. IIUC, Heiko 
means preempt_enable/disable is a single instruction on s390, but it is 
RMW on ARM64 (3 instructions for each).

Thanks,
Yang

>
> I am not sure what David Laight's code is supposed to do. Seems weird to
> me.
>