linux-kernel - Re: timerfd_settime/timerfd

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Date:	Wed, 30 Dec 2015 10:57:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Helge Deller <deller@....de>
cc:	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	linux-parisc <linux-parisc@...r.kernel.org>
Subject: Re: timerfd_settime/timerfd_gettime issue ?

On Tue, 29 Dec 2015, Helge Deller wrote:
> No, the patch below doesn't help.
> 
> I still see:
> [  644.916000] timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000) 
> [  645.024000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949) 
> 

Right. It can't help. Sorry for the distraction.

Looking deeper I found the issue. It's caused by CONFIG_TIME_LOW_RES. See the
comment in hrtimer_start_range_ns(). We round the expiry time to the next
jiffies period to avoid short timeouts. Assuming you are running with HZ=250
this is exactly 4ms. So that's where your extra time comes from.

Not sure what to do about that.

Thanks,

	tglx

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