Re: [PATCH next v2 2/5] locking/osq_lock: Optimise the vcpu_is_preempted() check.

[Ingo Molnar]

* David Laight <David.Laight@xxxxxxxxxx> 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