Re: [patch 18/39] tick: sched: Force tick interrupt and get rid of softirq magic

From: Frederic Weisbecker <fweisbec@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Preeti U Murthy <preeti@linux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Marcelo Tosatti <mtosatt@redhat.com>,
	John Stultz <john.stultz@linaro.org>,
	Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [patch 18/39] tick: sched: Force tick interrupt and get rid of softirq magic
Date: Wed, 22 Apr 2015 16:22:15 +0200	[thread overview]
Message-ID: <20150422142213.GA16832@lerouge> (raw)
In-Reply-To: <20150414203502.023464878@linutronix.de>

On Tue, Apr 14, 2015 at 09:08:54PM -0000, Thomas Gleixner wrote:
> We already got rid of the hrtimer reprogramming loops and hoops as
> hrtimer now enforces an interrupt if the enqueued time is in the past.
> 
> Do the same for the nohz non highres mode. That gets rid of the need
> to raise the softirq which only serves the purpose of getting the
> machine out of the inner idle loop.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> 
> ---
>  kernel/time/tick-sched.c |   83 ++++++++++++++++-------------------------------
>  1 file changed, 29 insertions(+), 54 deletions(-)
> 
> Index: tip/kernel/time/tick-sched.c
> ===================================================================
> --- tip.orig/kernel/time/tick-sched.c
> +++ tip/kernel/time/tick-sched.c
> @@ -565,6 +565,20 @@ u64 get_cpu_iowait_time_us(int cpu, u64
>  }
>  EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
>  
> +static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
> +{
> +	hrtimer_cancel(&ts->sched_timer);
> +	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
> +
> +	/* Forward the time to expire in the future */
> +	hrtimer_forward(&ts->sched_timer, now, tick_period);
> +
> +	if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
> +		hrtimer_start_expires(&ts->sched_timer, HRTIMER_MODE_ABS_PINNED);
> +	else
> +		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
> +}
> +
>  static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>  					 ktime_t now, int cpu)
>  {
> @@ -691,22 +705,18 @@ static ktime_t tick_nohz_stop_sched_tick
>  			if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
>  				hrtimer_cancel(&ts->sched_timer);
>  			goto out;
> -		}
> +		 }
>  
> -		if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
> -			hrtimer_start(&ts->sched_timer, expires,
> -				      HRTIMER_MODE_ABS_PINNED);
> -			goto out;
> -		} else if (!tick_program_event(expires, 0))
> -			goto out;
> -		/*
> -		 * We are past the event already. So we crossed a
> -		 * jiffie boundary. Update jiffies and raise the
> -		 * softirq.
> -		 */
> -		tick_do_update_jiffies64(ktime_get());
> +		 if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
> +			 hrtimer_start(&ts->sched_timer, expires,
> +				       HRTIMER_MODE_ABS_PINNED);
> +		 else
> +			 tick_program_event(expires, 1);
> +	} else {
> +		/* Tick is stopped, but required now. Enforce it */
> +		tick_nohz_restart(ts, now);
>  	}
> -	raise_softirq_irqoff(TIMER_SOFTIRQ);
> +

But the reprogramming happens only under "if ((long)delta_jiffies >= 1)".
Probably this condition should go away as well.

In the end, the possible side effect, at least on low-res, is that timers
which are already expired will be handled on the next tick instead of now.
But probably it doesn't matter much to have a one-tick delay.