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.
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