linux-kernel - Re: time / gtod seconds value out of sync?

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Date:	Thu, 19 Feb 2015 20:16:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>, jstancek@...hat.com
Subject: Re: time / gtod seconds value out of sync?


* John Stultz <john.stultz@...aro.org> wrote:

> > [  313.001823] NACC: timekeeping_get_ns = 1000121642
> > [  314.001889] NACC: timekeeping_get_ns = 188401
> >
> > gtod correctly accumulates those nsecs into the secs value:
> >
> >                 ts->tv_sec = tk->xtime_sec;
> >                 nsecs = timekeeping_get_ns(&tk->tkr);
> >                 ts->tv_nsec = 0;
> >                 timespec64_add_ns(ts, nsecs);
> >
> > but time() does:
> >
> >                 return tk->xtime_sec;
> >
> > It seems like overkill to do the full timekeeping_get_ns() in time(),
> 
> Right, so looking into the git history, 
> f20bf6125605acbbc7eb8c9420d7221c91aa83eb (time: introduce 
> xtime_seconds) changed this specifically for performance 
> reasons (cc'ed Ingo here, in case he remembers more 
> context).
> 
> The idea that time() would be ok as being HZ granular, 
> and its been this way since 2.6.23.  Thus you have a < HZ 
> sized window where gettimeofday() will return the next 
> second before time() gets updated by the tick.

Yes, and the scalability advantage is significant if you 
have an app that calls time() often. Undoing that would 
certainly make me sad.

Thanks,

	Ingo
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