linux-kernel - Re: HRTimer causing rtctest to fail

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Message-ID: <alpine.DEB.2.21.1802281246420.1886@nanos.tec.linutronix.de>
Date:   Wed, 28 Feb 2018 12:50:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org
Subject: Re: HRTimer causing rtctest to fail

On Wed, 28 Feb 2018, Felipe Balbi wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> > All numbers in micro seconds! What happens if you load the system or
> > restrict C-States?
> 
> added intel_idle.max_cstate=0 to cmdline. Still see some failures,
> albeit a lot more rare, for example, caught one on iteration 8 of
> 8192Hz:
> 
> 8192Hz:	 1 2 3 4 5 6 7
> PIE delta error: 0.000135 should be close to 0.000122
> 
> That is, however, only 13 uS late. Is that within the limits of "we
> don't quite care"?

I think so. 13us (depending on the machine) can easily happen if multiple
interrupts/timers happen at the same time or if something keeps interrupts
disabled for a few microseconds which is certainly possible.

What kind of system is this?

> BTW, it took some effort to get that one failure. I'll leave rtctest
> running in a long loop and try to figure out approximately how frequently
> it fails, but it seems like it's within the realm of "don't care", would
> you agree?

Yes.

Thanks,

	tglx