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

Ingo Molnar
2024-01-02T09:49:36Z

* David Laight <David.Laight@ACULAB.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