linux-kernel - Re: [patch 5/8] ntp: Make the RTC synchronization more reliable

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Message-ID: <20201207124749.GH2352378@localhost>
Date:   Mon, 7 Dec 2020 13:47:49 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 5/8] ntp: Make the RTC synchronization more reliable

On Sun, Dec 06, 2020 at 10:46:18PM +0100, Thomas Gleixner wrote:
> Switch it to an hrtimer instead which schedules the actual update work. The
> hrtimer will expire precisely (max 1 jiffie delay when high resolution
> timers are not available). The actual scheduling delay of the work is the
> same as before.

It works well in my tests.

> This becomes now:
> 
>      if (ntp_synced() && !hrtimer_is_queued(&sync_hrtimer))
>      	queue_work(system_power_efficient_wq, &sync_work, 0);
> 
> which is racy when the hrtimer has expired and the work is currently
> executed and has not yet managed to rearm the hrtimer.
> 
> Not a big problem as it just schedules work for nothing.

No more unexpected updates of the RTC observed.

Tested-by: Miroslav Lichvar <mlichvar@...hat.com>

Thanks,

-- 
Miroslav Lichvar