linux-kernel - Re: [RFC PATCH 1/4] Minimally invasive deadline wrap-around bugfix for SCHED

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Date:	Mon, 1 Aug 2016 12:43:55 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Cc:	Luca Abeni <luca.abeni@...tn.it>,
	Juri Lelli <juri.lelli@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-dl@...is.sssup.it
Subject: Re: [RFC PATCH 1/4] Minimally invasive deadline wrap-around bugfix
 for SCHED_DEADLINE cpu heap

Hi,

On 19/07/16 11:44, Tommaso Cucinotta wrote:
> Current code in cpudeadline.c has a bug in re-heapifying when adding a
> new element at the end of the heap, because a deadline value of 0 is
> temporarily set in the new elem, then cpudl_change_key() is called
> with the actual elem deadline as param. However, the function compares
> the new deadline to set with the one previously in the elem, which is
> 0.  So, if current absolute deadlines grew so much to have negative
> values as s64, the comparison in cpudl_change_key() makes the wrong
> decision.  Instead, as from dl_time_before(), the kernel should handle
> correctly abs deadlines wrap-arounds.
> 
> This patch fixes the problem with a minimally invasive change that
> forces cpudl_change_key() to heapify up in this case.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@....com>
> Cc: Luca Abeni <luca.abeni@...tn.it>
> Reviewed-by: Luca Abeni <luca.abeni@...tn.it>
> Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
> ---
>  kernel/sched/cpudeadline.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 5be5882..d418449 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -168,7 +168,7 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl, int is_valid)
>  
>  	if (old_idx == IDX_INVALID) {
>  		cp->size++;
> -		cp->elements[cp->size - 1].dl = 0;
> +		cp->elements[cp->size - 1].dl = dl;
>  		cp->elements[cp->size - 1].cpu = cpu;
>  		cp->elements[cpu].idx = cp->size - 1;
>  		cpudl_change_key(cp, cp->size - 1, dl);

Looks good. I'd only change the subject to something like:

 sched/deadline: Wrap-around bugfix for cpu heap

Best,

- Juri