Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Shrikanth Hegde]

On 7/27/26 3:43 PM, Jirka Hladky wrote:
> Hi Shrikanth,

[...]

> The real question is: why does enabling PREEMPT_RCU cost ~33% on
> ppc64le but only ~3% on x86_64? The answer is that x86_64's TSO
> memory model makes the barriers in __rcu_read_lock/__rcu_read_unlock
> essentially free, while ppc64le's weak ordering requires explicit
> lwsync/isync instructions, which are expensive when called thousands
> of times per second in the SELinux AVC hot path.

Ensure when comparing the preemption modes are same or not.
x86 has dynamic preemption since long time. It might be possible in your
testing when you compared for x86, preemption modes were same, whereas in
case of powerpc you are comparing between voluntary vs lazy.

If it is voluntary vs lazy, this would simplify where to concentrate.
(You can easily do this with CONFIG_PREEMPT_DYNAMIC=n on a kernel before 7.0)