From: Marcelo Tosatti <mtosatti@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Xu <peterx@redhat.com>,
Anna-Maria Behnsen <anna-maria@linutronix.de>,
linux-kernel@vger.kernel.org,
Frederic Weisbecker <frederic@kernel.org>,
Nitesh Narayan Lal <nitesh@redhat.com>,
Alex Belits <abelits@marvell.com>
Subject: Re: [PATCH v5] hrtimer: avoid retrigger_next_event IPI
Date: Mon, 19 Apr 2021 15:56:19 -0300 [thread overview]
Message-ID: <20210419185619.GA57245@fuller.cnet> (raw)
In-Reply-To: <87mttwsvlv.ffs@nanos.tec.linutronix.de>
On Sat, Apr 17, 2021 at 06:51:08PM +0200, Thomas Gleixner wrote:
> On Sat, Apr 17 2021 at 18:24, Thomas Gleixner wrote:
> > On Fri, Apr 16 2021 at 13:13, Peter Xu wrote:
> >> On Fri, Apr 16, 2021 at 01:00:23PM -0300, Marcelo Tosatti wrote:
> >>>
> >>> +#define CLOCK_SET_BASES ((1U << HRTIMER_BASE_REALTIME) | \
> >>> + (1U << HRTIMER_BASE_REALTIME_SOFT) | \
> >>> + (1U << HRTIMER_BASE_TAI) | \
> >>> + (1U << HRTIMER_BASE_TAI_SOFT))
> >>> +
> >>> +static bool need_reprogram_timer(struct hrtimer_cpu_base *cpu_base)
> >>> +{
> >>> + if (cpu_base->softirq_activated)
> >>> + return true;
> >>
> >> A pure question on whether this check is needed...
> >>
> >> Here even if softirq_activated==1 (as softirq is going to happen), as long as
> >> (cpu_base->active_bases & CLOCK_SET_BASES)==0, shouldn't it already mean that
> >> "yes indeed clock was set, but no need to kick this cpu as no relevant timer"?
> >> As that question seems to be orthogonal to whether a softirq is going to
> >> trigger on that cpu.
> >
> > That's correct and it's not any different from firing the IPI because in
> > both cases the update happens with the base lock of the CPU in question
> > held. And if there are no active timers in any of the affected bases,
> > then there is no need to reevaluate the next expiry because the offset
> > update does not affect any armed timers. It just makes sure that the
> > next enqueu of a timer on such a base will see the the correct offset.
> >
> > I'll just zap it.
>
> But the whole thing is still wrong in two aspects:
>
> 1) BOOTTIME can be one of the affected clocks when sleep time
> (suspended time) is injected because that uses the same mechanism.
>
> Sorry for missing that earlier when I asked to remove it, but
> that's trivial to fix by adding the BOOTTIME base back.
>
> 2) What's worse is that on resume this might break because that
> mechanism is also used to enforce the reprogramming of the clock
> event devices and there we cannot be selective on clock bases.
>
> I need to dig deeper into that because suspend/resume has changed
> a lot over time, so this might be just a historical leftover. But
> without proper analysis we might end up with subtle and hard to
> debug wreckage.
>
> Thanks,
>
> tglx
Thomas,
There is no gain in avoiding the IPIs for the suspend/resume case
(since suspending is a large interruption anyway). To avoid
the potential complexity (and associated bugs), one option would
be to NOT skip IPIs for the resume case.
Sending -v6 with that (and other suggestions/fixes).
next prev parent reply other threads:[~2021-04-19 19:40 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
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 [this message]
2021-04-19 19:39 ` [PATCH v6] " Marcelo Tosatti
2021-04-19 20:52 ` Thomas Gleixner
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