From: Marcelo Tosatti <mtosatti@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
Frederic Weisbecker <frederic@kernel.org>,
Peter Xu <peterx@redhat.com>,
Nitesh Narayan Lal <nitesh@redhat.com>,
Alex Belits <abelits@marvell.com>
Subject: Re: [PATCH] hrtimer: avoid retrigger_next_event IPI
Date: Fri, 9 Apr 2021 13:51:46 -0300 [thread overview]
Message-ID: <20210409165146.GA40118@fuller.cnet> (raw)
In-Reply-To: <87o8en4k9a.ffs@nanos.tec.linutronix.de>
+CC Anna-Maria.
On Fri, Apr 09, 2021 at 04:15:13PM +0200, Thomas Gleixner wrote:
> On Wed, Apr 07 2021 at 10:53, Marcelo Tosatti wrote:
> > Setting the realtime clock triggers an IPI to all CPUs to reprogram
> > hrtimers.
> >
> > However, only base, boottime and tai clocks have their offsets updated
>
> base clock?
Heh...
> And why boottime? Boottime is not affected by a clock
> realtime set. It's clock REALTIME and TAI, nothing else.
OK!
> > +#define CLOCK_SET_BASES ((1U << HRTIMER_BASE_REALTIME)| \
> > + (1U << HRTIMER_BASE_REALTIME_SOFT)| \
> > + (1U << HRTIMER_BASE_BOOTTIME)| \
> > + (1U << HRTIMER_BASE_BOOTTIME_SOFT)| \
> > + (1U << HRTIMER_BASE_TAI)| \
> > + (1U << HRTIMER_BASE_TAI_SOFT))
> > +
> > +static bool need_reprogram_timer(struct hrtimer_cpu_base *cpu_base)
> > +{
> > + unsigned int active = 0;
> > +
> > + if (!cpu_base->softirq_activated)
> > + active = cpu_base->active_bases & HRTIMER_ACTIVE_SOFT;
Again, if (cpu_base->softirq_activated), need to IPI (will resend).
> > + active = active | (cpu_base->active_bases & HRTIMER_ACTIVE_HARD);
> > +
> > + if ((active & CLOCK_SET_BASES) == 0)
> > + return false;
> > +
> > + return true;
> > +}
>
> Errm.
What?
> > + /* Avoid interrupting nohz_full CPUs if possible */
> > + preempt_disable();
> > + for_each_online_cpu(cpu) {
> > + if (tick_nohz_full_cpu(cpu)) {
> > + unsigned long flags;
> > + struct hrtimer_cpu_base *cpu_base = &per_cpu(hrtimer_bases, cpu);
> > +
> > + raw_spin_lock_irqsave(&cpu_base->lock, flags);
> > + if (need_reprogram_timer(cpu_base))
> > + cpumask_set_cpu(cpu, mask);
> > + else
> > + hrtimer_update_base(cpu_base);
> > + raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
> > + }
> > + }
>
> How is that supposed to be correct?
>
> CPU0 CPU1
>
> clock_was_set() hrtimer_start(CLOCK_REALTIME)
>
> if (!active_mask[CPU1] & XXX)
> continue;
> active_mask |= REALTIME;
>
> ---> fail because that newly started timer is on the old offset.
CPU0 CPU1
clock_was_set()
Case-1: CPU-1 grabs base->lock before CPU-0:
CPU-0 sees active_mask[CPU1] and IPIs.
base = lock_hrtimer_base(timer, &flags);
if (__hrtimer_start_range_ns(timer, tim, ...
hrtimer_reprogram(timer, true);
unlock_hrtimer_base(timer, &flags);
raw_spin_lock_irqsave(&cpu_base->lock, flags);
if (need_reprogram_timer(cpu_base))
cpumask_set_cpu(cpu, mask);
else
hrtimer_update_base(cpu_base);
raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
Case-2: CPU-1 grabs base->lock after CPU-0:
CPU-0 will have updated the offsets remotely.
base = lock_hrtimer_base(timer, &flags);
if (__hrtimer_start_range_ns(timer, tim, ...
hrtimer_reprogram(timer, true);
unlock_hrtimer_base(timer, &flags);
No?
next prev parent reply other threads:[~2021-04-09 16:52 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-07 13:53 [PATCH] hrtimer: avoid retrigger_next_event IPI Marcelo Tosatti
2021-04-07 19:28 ` kernel test robot
2021-04-07 22:14 ` Frederic Weisbecker
2021-04-08 12:27 ` Marcelo Tosatti
2021-04-09 14:15 ` Thomas Gleixner
2021-04-09 16:51 ` Marcelo Tosatti [this message]
2021-04-10 7:53 ` Thomas Gleixner
2021-04-13 17:04 ` [PATCH v2] " Marcelo Tosatti
2021-04-14 17:19 ` Thomas Gleixner
2021-04-15 15:39 ` [PATCH v3] " Marcelo Tosatti
2021-04-15 18:59 ` Thomas Gleixner
2021-04-15 20:40 ` [PATCH v4] " Marcelo Tosatti
2021-04-16 16:00 ` [PATCH v5] " Marcelo Tosatti
2021-04-16 17:13 ` Peter Xu
2021-04-17 16:24 ` Thomas Gleixner
2021-04-17 16:51 ` Thomas Gleixner
2021-04-19 18:56 ` Marcelo Tosatti
2021-04-19 19:39 ` [PATCH v6] " Marcelo Tosatti
2021-04-19 20:52 ` Thomas Gleixner
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