linux-kernel - Re: [PATCH] sched/core: fix sched_rt_global

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Date:	Mon, 3 Feb 2014 14:31:02 +0100
From:	Henrik Austad <henrik@...tad.us>
To:	Juri Lelli <juri.lelli@...il.com>
Cc:	peterz@...radead.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: fix sched_rt_global_validate

On Mon, Feb 03, 2014 at 11:14:05AM +0100, Juri Lelli wrote:
> On 02/03/2014 10:53 AM, Henrik Austad wrote:
> > On Fri, Jan 31, 2014 at 05:43:27PM +0100, Juri Lelli wrote:
> >> Don't compare sysctl_sched_rt_runtime against sysctl_sched_rt_period if
> >> the former is equal to RUNTIME_INF, otherwise disabling -rt bandwidth
> >> management always fails.
> >>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Signed-off-by: Juri Lelli <juri.lelli@...il.com>
> >> ---
> >>  kernel/sched/core.c |    3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >> index 210a12a..5c0a304 100644
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -7477,7 +7477,8 @@ static int sched_rt_global_validate(void)
> >>  	if (sysctl_sched_rt_period <= 0)
> >>  		return -EINVAL;
> >>  
> >> -	if (sysctl_sched_rt_runtime > sysctl_sched_rt_period)
> >> +	if ((sysctl_sched_rt_runtime != RUNTIME_INF) &&
> >> +		(sysctl_sched_rt_runtime > sysctl_sched_rt_period))
> >>  		return -EINVAL;
> > 
> > Won't this be caught by the test above?
> > 
> > #define RUNTIME_INF ((u64)~0ULL)
> > 
> > which means that if sysctl_sched_rt_runtime is set to RUNTIME_INF, it will 
> > trigger on the previous test, and the first part of this test will always 
> > be true.
> > 
> > Or have I suffered catastrophic monday-morning braindamage?
> > 
> 
> As I understand it. When you do
> 
>  echo -1 > /proc/sys/kernel/sched_rt_runtime_us
> 
> sysctl_sched_rt_runtime is actually set to -1 (being an int).

Yes.

> But then you compare it against and unsigned int, so the cast converts it to
> actually be RUNTIME_INF, and thus greater than sysctl_sched_rt_period (so the
> function returns -EINVAL, while you'd want it to return 0, as you are disabling
> -rt throttling).

Ah, yes, it comes down to my early-monday brain hemorrhage, mixing up 
rt_period and rt_runtime. My apologies!

> Makes sense?

Yes it does.

-- 
Henrik Austad

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