linux-kernel - Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the vcpu_is

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Date: Tue, 2 Jan 2024 10:49:32 +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>,
	"'virtualization@...ts.linux-foundation.org'" <virtualization@...ts.linux-foundation.org>,
	'Zeng Heng' <zengheng4@...wei.com>
Subject: Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the
 vcpu_is_preempted() check.


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

> The vcpu_is_preempted() test stops osq_lock() spinning if a virtual
> cpu is no longer running.
> 
> Although patched out for bare-metal the code still needs the cpu number.

Comma missing.

> Reading this from 'prev->cpu' is a pretty much guaranteed have a cache miss
> when osq_unlock() is waking up the next cpu.
> 
> Instead save 'prev->cpu' in 'node->prev_cpu' and use that value instead.
> Update in the osq_lock() 'unqueue' path when 'node->prev' is changed.
> 
> This is simpler than checking for 'node->prev' changing and caching
> 'prev->cpu'.

Throughout the series, in changelogs and comments, please do:

 s/cpu
  /CPU

Please be more careful about changelog quality.

>  struct optimistic_spin_node {
>  	struct optimistic_spin_node *next, *prev;
> -	int locked; /* 1 if lock acquired */
> -	int cpu; /* encoded CPU # + 1 value */
> +	int locked;    /* 1 if lock acquired */
> +	int cpu;       /* encoded CPU # + 1 value */
> +	int prev_cpu;  /* encoded CPU # + 1 value */
>  };

s/ encoded CPU # + 1 value
 / encoded value: CPU+1

Thanks,

	Ingo