Re: [PATCH 3/3] cpuidle: powernv: Avoid a branch in the core snooze_loop() loop

[Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>]

* Anton Blanchard <anton@ozlabs.org> [2017-04-04 07:54:14]:

> From: Anton Blanchard <anton@samba.org>
> 
> When in the snooze_loop() we want to take up the least amount of
> resources. On my version of gcc (6.3), we end up with an extra
> branch because it predicts snooze_timeout_en to be false, whereas it
> is almost always true.

By default snooze_timeout_en is true.  It will become false only when
we do not want to exit the snooze loop and that will be when snooze is
the only idle state available in the platform, which is a rare case.

> Use likely() to avoid the branch and be a little nicer to the
> other non idle threads on the core.
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>

Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>

> ---
>  drivers/cpuidle/cpuidle-powernv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c
> index 8c991c254b95..251a60bfa8ee 100644
> --- a/drivers/cpuidle/cpuidle-powernv.c
> +++ b/drivers/cpuidle/cpuidle-powernv.c
> @@ -58,7 +58,7 @@ static int snooze_loop(struct cpuidle_device *dev,
>  	ppc64_runlatch_off();
>  	HMT_very_low();
>  	while (!need_resched()) {
> -		if (snooze_timeout_en && get_tb() > snooze_exit_time)
> +		if (likely(snooze_timeout_en) && get_tb() > snooze_exit_time)
>  			break;
>  	}
> 
> -- 
> 2.11.0
>