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

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Date: Sat, 30 Dec 2023 21:37:32 +0100
From: Ingo Molnar <mingo@...nel.org>
To: David Laight <David.Laight@...lab.com>
Cc: "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'peterz@...radead.org'" <peterz@...radead.org>,
	"'longman@...hat.com'" <longman@...hat.com>,
	"'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.


* David Laight <David.Laight@...LAB.COM> wrote:

>  bool osq_lock(struct optimistic_spin_queue *lock)
>  {
> -	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> +	struct optimistic_spin_node *node = raw_cpu_read(osq_node.self);
>  	struct optimistic_spin_node *prev, *next;
>  	int old;
>  
> -	if (unlikely(node->cpu == OSQ_UNLOCKED_VAL))
> -		node->cpu = encode_cpu(smp_processor_id());
> +	if (unlikely(!node)) {
> +		int cpu = encode_cpu(smp_processor_id());
> +		node = decode_cpu(cpu);
> +		node->self = node;
> +		node->cpu = cpu;

This whole initialization sequence is suboptimal and needs to be 
cleaned up first: the node->cpu field is constant once initialized, so 
it should be initialized from appropriate init methods, not runtime in 
osq_lock(), right?

Eliminating that initialization branch is a useful micro-optimization 
in itself for the hot path.

Thanks,

	Ingo