Re: [PATCH v2 13/20] arm64: percpu: Add infrastructure for preemptible this_cpu_*() ops

["David Hildenbrand (Arm)" <david@kernel.org>]

> Atop v7.2-rc4, with GCC 15.2.0 and defconfig, this is compiled as:
> 
> | <outline_this_cpu_add_u64>:
> |        paciasp
> |        stp     x29, x30, [sp, #-16]!
> |        mrs     x2, sp_el0
> |        mov     x29, sp
> |        ldr     w3, [x2, #8]
> |        add     w3, w3, #0x1
> |        str     w3, [x2, #8]
> |        mrs     x3, tpidr_el1
> |        add     x0, x0, x3
> | 1:     ldxr    x5, [x0]
> |        add     x5, x5, x1
> |        stxr    w4, x5, [x0]
> |        cbnz    w4, 1b
> |        ldr     x0, [x2, #8]
> |        sub     x0, x0, #0x1
> |        str     w0, [x2, #8]
> |        cbz     x0, 2f
> |        ldr     x0, [x2, #8]
> |        cbnz    x0, 3f
> | 2:     bl      preempt_schedule_notrace
> | 3:     ldp     x29, x30, [sp], #16
> |        autiasp
> |        ret

FWIW, in a recent discussion on some prototype hacking [1] we saw some overhead
in micro-benchmarks that would really hammer on a path that would now do a
preempt_disable()+preempt_enable().

Switching from preempt_disable() to preempt_enable_no_resched() made it turn to
noise. Of course, that has other undesirable impacts, and I am not sure if we
are in the territory of code layout changes affecting the numbers.

Just mentioning it as some data point.

-- 
Cheers,

David