[RFC][PATCH] sched/deadline: Remove if statement before clearing throttle and yielded

[RFC][PATCH] sched/deadline: Remove if statement before clearing throttle and yielded

[Steven Rostedt]

[
  This is an RFC as I didn't run any benchmarks. It just seemed a bit 
  weird to me that we would add such a check instead of just clearing
  these variables out regardless.
]

The function replenish_dl_entity() clears dl_throttled and dl_yielded,
but checks first if they are set before doing so. As these variables
are in the same cache locale of other variables being modified, there's
no advantage in checking if they are set before clearing them. But
having the compare takes slots away from the branch prediction.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a2ce590..9748d33 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -423,10 +423,8 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se,
 		dl_se->runtime = pi_se->dl_runtime;
 	}
 
-	if (dl_se->dl_yielded)
-		dl_se->dl_yielded = 0;
-	if (dl_se->dl_throttled)
-		dl_se->dl_throttled = 0;
+	dl_se->dl_yielded = 0;
+	dl_se->dl_throttled = 0;
 }
 
 /*

Re: [RFC][PATCH] sched/deadline: Remove if statement before clearing throttle and yielded

[Juri Lelli]

Hi,

On 10/05/17 09:50, Steven Rostedt wrote:
> [
>   This is an RFC as I didn't run any benchmarks. It just seemed a bit 
>   weird to me that we would add such a check instead of just clearing
>   these variables out regardless.
> ]
> 
> The function replenish_dl_entity() clears dl_throttled and dl_yielded,
> but checks first if they are set before doing so. As these variables
> are in the same cache locale of other variables being modified, there's
> no advantage in checking if they are set before clearing them. But
> having the compare takes slots away from the branch prediction.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index a2ce590..9748d33 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -423,10 +423,8 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se,
>  		dl_se->runtime = pi_se->dl_runtime;
>  	}
>  
> -	if (dl_se->dl_yielded)
> -		dl_se->dl_yielded = 0;
> -	if (dl_se->dl_throttled)
> -		dl_se->dl_throttled = 0;
> +	dl_se->dl_yielded = 0;
> +	dl_se->dl_throttled = 0;
>  }

Looks good to me.

Peter, any particular reason why you wanted to first check the values?

Best,

- Juri

Re: [RFC][PATCH] sched/deadline: Remove if statement before clearing throttle and yielded

[Daniel Bristot de Oliveira]

On 05/11/2017 04:01 PM, Juri Lelli wrote:
> Looks good to me.
> 
> Peter, any particular reason why you wanted to first check the values?

+1

-- Daniel

Re: [RFC][PATCH] sched/deadline: Remove if statement before clearing throttle and yielded

[Peter Zijlstra]

On Thu, May 11, 2017 at 03:01:16PM +0100, Juri Lelli wrote:

> > -	if (dl_se->dl_yielded)
> > -		dl_se->dl_yielded = 0;
> > -	if (dl_se->dl_throttled)
> > -		dl_se->dl_throttled = 0;
> > +	dl_se->dl_yielded = 0;
> > +	dl_se->dl_throttled = 0;
> >  }
> 
> Looks good to me.
> 
> Peter, any particular reason why you wanted to first check the values?

No idea, could be general paranoia on unconditional writes or something.