Re: [PATCH 4/8] nohz: Remove idle task special case

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 06/11/2015 11:06 PM, Frederic Weisbecker wrote:
> This is a leftover from old days to avoid conflicts with dynticks idle
> code. Now full dynticks and idle dynticks are better integrated and
> interact without known issue.

I am sorry but I fail to understand why the check on idle task was there
in the first place in the below code paths. It would help if you could
clarify this in the changelog as well.

> 
> So lets remove it.
> 
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc; John Stultz <john.stultz@linaro.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  kernel/time/tick-sched.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 812f7a3..324482f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -208,10 +208,8 @@ void __tick_nohz_full_check(void)
>  	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
> 
>  	if (tick_nohz_full_cpu(smp_processor_id())) {
> -		if (ts->tick_stopped && !is_idle_task(current)) {
> -			if (!can_stop_full_tick())

can_stop_full_tick() would have bailed out if the current task was idle,
since it checks for the number of tasks being greater than 1 to restart
the tick. So why was the check is_idle_task() introduced earlier ?

> -				tick_nohz_restart_sched_tick(ts, ktime_get());
> -		}
> +		if (ts->tick_stopped && !can_stop_full_tick())
> +			tick_nohz_restart_sched_tick(ts, ktime_get());
>  	}
>  }
> 
> @@ -710,7 +708,7 @@ static void tick_nohz_full_stop_tick(struct tick_sched *ts)
>  #ifdef CONFIG_NO_HZ_FULL
>  	int cpu = smp_processor_id();
> 
> -	if (!tick_nohz_full_cpu(cpu) || is_idle_task(current))
> +	if (!tick_nohz_full_cpu(cpu))

If the current task was indeed idle, the check on ts->inidle would have
succeeded in tick_irq_exit() and we would not have reached this function
at all, isn't it? So here too I am unable to understand why we had it in
the first place.

Regards
Preeti U Murthy
>  		return;
> 
>  	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
>