Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency mask model

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Aug 10, 2015 at 05:11:51PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 10, 2015 at 04:28:47PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 10, 2015 at 04:16:58PM +0200, Frederic Weisbecker wrote:
> > 
> > > I considered many times relying on hrtick btw but everyone seem to say it has a lot
> > > of overhead, especially due to clock reprogramming on schedule() calls.
> > 
> > Yeah, I have some vague ideas of how to take out much of that overhead
> > (tglx will launch frozen sharks at me I suspect), but we cannot get
> > around the overhead of actually having to program the hardware and that
> > is still a significant amount on many machines.
> > 
> > Supposedly machines with TSC deadline are better, but I've not tried
> > to benchmark that.
> 
> Basically something along these lines.. which avoids a whole bunch of
> hrtimer stuff.
> 
> But without fast hardware its all still pointless.
> 
> diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
> index 76dd4f0da5ca..c279950cb8c3 100644
> --- a/include/linux/hrtimer.h
> +++ b/include/linux/hrtimer.h
> @@ -200,6 +200,7 @@ struct hrtimer_cpu_base {
>  	unsigned int			nr_retries;
>  	unsigned int			nr_hangs;
>  	unsigned int			max_hang_time;
> +	ktime_t				expires_sched;
>  #endif
>  	struct hrtimer_clock_base	clock_base[HRTIMER_MAX_CLOCK_BASES];
>  } ____cacheline_aligned;
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 5c7ae4b641c4..be9c0a555eaa 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -68,6 +68,7 @@ DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) =
>  {
>  	.lock = __RAW_SPIN_LOCK_UNLOCKED(hrtimer_bases.lock),
>  	.seq = SEQCNT_ZERO(hrtimer_bases.seq),
> +	.expires_sched = { .tv64 = KTIME_MAX, },
>  	.clock_base =
>  	{
>  		{
> @@ -460,7 +461,7 @@ static inline void hrtimer_update_next_timer(struct hrtimer_cpu_base *cpu_base,
>  static ktime_t __hrtimer_get_next_event(struct hrtimer_cpu_base *cpu_base)
>  {
>  	struct hrtimer_clock_base *base = cpu_base->clock_base;
> -	ktime_t expires, expires_next = { .tv64 = KTIME_MAX };
> +	ktime_t expires, expires_next = cpu_base->expires_sched;
>  	unsigned int active = cpu_base->active_bases;
>  
>  	hrtimer_update_next_timer(cpu_base, NULL);
> @@ -1289,6 +1290,33 @@ static void __hrtimer_run_queues(struct hrtimer_cpu_base *cpu_base, ktime_t now)
>  
>  #ifdef CONFIG_HIGH_RES_TIMERS
>  
> +void sched_hrtick_set(u64 ns)
> +{
> +	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
> +	ktime_t expires = ktime_add_ns(ktime_get(), ns);
> +
> +	raw_spin_lock(&cpu_base->lock);
> +	cpu_base->expires_sched = expires;
> +
> +	if (expires.tv64 < cpu_base->expires_next.tv64)
> +		hrtimer_force_reprogram(cpu_base, 0);
> +
> +	raw_spin_unlock(&cpu_base->lock);
> +}
> +
> +void sched_hrtick_cancel(void)
> +{
> +	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
> +
> +	raw_spin_lock(&cpu_base->lock);
> +	/*
> +	 * If the current event was this sched event, eat the superfluous
> +	 * interrupt rather than touch the hardware again.
> +	 */
> +	cpu_base->expires_sched.tv64 = KTIME_MAX;
> +	raw_spin_unlock(&cpu_base->lock);
> +}

Well, there could be a more proper way to do this without tying that to the scheduler
tick. This could be some sort of hrtimer_cancel_soft() which more generally cancels a
timer without cancelling the interrupt itself. We might want to still keep track of that
lost interrupt though in case of later clock reprogramming that fits the lost interrupt.
With a field like cpu_base->expires_interrupt. I thought about expires_soft and expires_hard
but I think that terminology is already used :-)

That said that feature at least wouldn't fit nohz full which really wants to avoid spurious
interrupts.