linux-kernel - Re: [PATCH 1/2] sched/nohz: add debugfs control over sched_tick_max

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Message-ID: <20140105132153.GA25212@localhost.localdomain>
Date:	Sun, 5 Jan 2014 14:21:55 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	linaro-kernel@...ts.linaro.org, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] sched/nohz: add debugfs control over
 sched_tick_max_deferment

On Tue, Dec 17, 2013 at 01:23:07PM -0800, Kevin Hilman wrote:
> Allow debugfs override of sched_tick_max_deferment in order to ease
> finding/fixing the remaining issues with full nohz.
> 
> The value to be written is in jiffies, and -1 means the max deferment
> is disabled (scheduler_tick_max_deferment() returns KTIME_MAX.)
> 
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Signed-off-by: Kevin Hilman <khilman@...aro.org>
> ---
>  kernel/sched/core.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5ac63c9a995a..4b1fe3e69fe4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2175,6 +2175,8 @@ void scheduler_tick(void)
>  }
>  
>  #ifdef CONFIG_NO_HZ_FULL
> +static u32 sched_tick_max_deferment = HZ;
> +
>  /**
>   * scheduler_tick_max_deferment
>   *
> @@ -2193,13 +2195,25 @@ u64 scheduler_tick_max_deferment(void)
>  	struct rq *rq = this_rq();
>  	unsigned long next, now = ACCESS_ONCE(jiffies);
>  
> -	next = rq->last_sched_tick + HZ;
> +	if (sched_tick_max_deferment == -1)
> +		return KTIME_MAX;
> +
> +	next = rq->last_sched_tick + sched_tick_max_deferment;
>  
>  	if (time_before_eq(next, now))
>  		return 0;
>  
>  	return jiffies_to_usecs(next - now) * NSEC_PER_USEC;
>  }
> +
> +static __init int sched_nohz_full_init_debug(void)
> +{
> +	debugfs_create_u32("sched_tick_max_deferment", 0644, NULL,
> +			   &sched_tick_max_deferment);
> +
> +	return 0;
> +}
> +late_initcall(sched_nohz_full_init_debug);

If the goal is mostly to turn off sched_tick_max_deferment (set to -1), we should
perhaps make it a boolean sched feature (see kernel/sched/features.h) as it's a pretty
well consolidated interface.

>  #endif
>  
>  notrace unsigned long get_parent_ip(unsigned long addr)
> -- 
> 1.8.3
> 
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