RE: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data accesses.

[David Laight]

From: Waiman Long
> Sent: 31 December 2023 03:04
....
> The presence of debug_smp_processor_id in your compiled code is likely
> due to the setting of CONFIG_DEBUG_PREEMPT in your kernel config.
> 
> #ifdef CONFIG_DEBUG_PREEMPT
>    extern unsigned int debug_smp_processor_id(void);
> # define smp_processor_id() debug_smp_processor_id()
> #else
> # define smp_processor_id() __smp_processor_id()
> #endif
> 
> I don't have that config entry in my kernel config and so I only get 2
> instructions for this_cpu_ptr(). We are not going to optimize the code
> specifically for CONFIG_DEBUG_PREEMPT and so this patch should be dropped.

Yes, I discovered that late last night.
I've no idea why it is set.
It might even be inherited from the ubuntu 18.04 (I think) .config
I started with (mostly removing extra modules to reduce compile
time and the size of uramdisk).

I'll look at the patches again.
Saving node->prev->cpu as node->prev_cpu will definitely save
the cache-line bounce.

	David

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