linux-kernel - Re: [PATCH] time, ntp: Do not update time

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Message-ID: <20150219170048.GB25948@midget.suse.cz>
Date:	Thu, 19 Feb 2015 18:00:48 +0100
From:	Jiri Bohac <jbohac@...e.cz>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] time, ntp: Do not update time_state in middle of leap
 second [v3]

Hi,

I'm trying to understand what exactly is going on here...

On Thu, Feb 12, 2015 at 08:58:19AM -0500, Prarit Bhargava wrote:
> The test did the following in userspace:
> 
>         tx.modes = ADJ_STATUS;
>         tx.status = STA_INS;
> 
> 	/* send leap second request */
>         ret = adjtimex(&tx);
> 
>         /* Check adjtimex output every half second */
>         now = tx.time.tv_sec;
>         while (now < next_leap+2) {
>                 char buf[26];
>                 ret = adjtimex(&tx);
> 
>                 ctime_r(&tx.time.tv_sec, buf);
>                 buf[strlen(buf)-1] = 0; /*remove trailing\n */
> 
>                 printf("%s + %6ld us\t%s\n",
>                                 buf,
>                                 tx.time.tv_usec,
>                                 time_state_str(ret));
>                 now = tx.time.tv_sec;
>                 /* Sleep for another half second */
>                 ts.tv_sec = 0;
>                 ts.tv_nsec = NSEC_PER_SEC/2;
>                 clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, NULL);
>         }
> 
> After some investigation it was noted that the test contained a small error:
> the test does not reinitialize tx.status and reissues the STA_INS every
> 1/2 second.

Prarit, can you explain who sets the STA_PLL flag, so that
process_adj_status() detects a STA_PLL->!STA_PLL transition and
goes to the branch that sets time_state = TIME_OK?

Is that ntpd running in parallel with your test program?  If that
is the case, you would eventually end up oscilating between the
the TIME_INS and TIME_OK states anyway, even with your patch.
ntpd will clear the STA_INS flag after midnight, the state
machine will transition from TIME_WAIT to TIME_OK and your test
program will set STA_INS again (fighting with ntpd which will
clear the flag from time to time) ...  right?

Thanks,

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, SUSE CZ

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