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

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Date: Sun, 31 Dec 2023 10:36:42 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Waiman Long' <longman@...hat.com>, 'Ingo Molnar' <mingo@...nel.org>
CC: "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'peterz@...radead.org'" <peterz@...radead.org>, "'mingo@...hat.com'"
	<mingo@...hat.com>, "'will@...nel.org'" <will@...nel.org>,
	"'boqun.feng@...il.com'" <boqun.feng@...il.com>, 'Linus Torvalds'
	<torvalds@...ux-foundation.org>, "'xinhui.pan@...ux.vnet.ibm.com'"
	<xinhui.pan@...ux.vnet.ibm.com>,
	"'virtualization@...ts.linux-foundation.org'"
	<virtualization@...ts.linux-foundation.org>, 'Zeng Heng'
	<zengheng4@...wei.com>
Subject: RE: [PATCH next 4/5] locking/osq_lock: Optimise per-cpu data
 accesses.

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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