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From: Imre Deak <imre.deak@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Ville Syrjälä" <ville.syrjala@linux.intel.com>,
	"Mika Kuoppala" <mika.kuoppala@intel.com>,
	"Chris Wilson" <chris@chris-wilson.co.uk>
Subject: Re: Early timeouts due to inaccurate jiffies during system suspend/resume
Date: Mon, 23 Apr 2018 20:01:28 +0300	[thread overview]
Message-ID: <20180423170128.mf7g26rniimm7asf@ideak-desk.fi.intel.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1804191256200.1563@nanos.tec.linutronix.de>

On Thu, Apr 19, 2018 at 01:05:39PM +0200, Thomas Gleixner wrote:
> On Thu, 19 Apr 2018, Imre Deak wrote:
> > Hi,
> > 
> > while checking bug [1], I noticed that jiffies based timing loops like
> > 
> > 	expire = jiffies + timeout + 1;
> > 	while (!time_after(jiffies, expire))
> > 		do_something;
> > 
> > can last shorter than expected (that is less than timeout).
> 
> Yes, that can happen when the timer interrupt is delayed long enough for
> whatever reason. If you need accurate timing then you need to use
> ktime_get().

Thanks. I always regarded jiffies as non-accurate, but something that
gives a minimum time delay guarantee (when adjusted by +1 as above). I
wonder if there are other callers in kernel that don't expect an early
timeout.

We switched now to using ktime_get_raw() in the i915 driver.

> 
> > After some ftracing it seems like jiffies gets stale due to a missed
> > LAPIC timer interrupt after the interrupt is armed in
> > lapic_next_deadline() and before jiffies is sampled at 2. above.
> > Eventually the interrupt does get delivered, at which point jiffies gets
> > updated via tick_do_update_jiffies64() with a >1 ticks increment.
> > Between lapic_next_deadline() and the - late - delivery of the interrupt
> > the CPU on which the interrupt is armed doesn't go idle.
> 
> That's odd. I have no real explanation for that.

Looks like the reason is IRQ latency. For reference here are the
longest ones I found with irqsoff ftracing, all running with IRQs disabled
during system resume:

hpet_rtc_interrupt()->hpet_rtc_timer_reinit():
do { ... } while(!hpet_cnt_ahead(...));
takes sometimes up to ~40msec for me.

hpet_rtc_interrupt()->mc146818_get_time():
if (mc146818_is_updating()) mdelay(20);

driver_probe_device->atkbd_connect()->i8042_port_close()->__i8042_command()->i8042_wait_write():
takes sometimes up to ~10msec for me.

All the above paired with asynchronous calling of the drivers' resume
hooks may result in the jumps in jiffies I saw.

--Imre

  reply	other threads:[~2018-04-23 17:01 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-19  1:32 Imre Deak
2018-04-19 11:05 ` Thomas Gleixner
2018-04-23 17:01   ` Imre Deak [this message]
2018-04-24 14:07     ` Imre Deak
2018-04-24 14:21       ` Ville Syrjälä
2018-04-26 21:40       ` Thomas Gleixner

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