linux-kernel - [PATCH v2 0/4] hv_util: adjust system time smoothly

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Date:   Wed,  4 Jan 2017 18:24:35 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     devel@...uxdriverproject.org
Cc:     linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alex Ng <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: [PATCH v2 0/4] hv_util: adjust system time smoothly

Changes since v1:
- do do_settimeofday64() when ICTIMESYNCFLAG_SYNC flag is present in the
  request (Alex Ng)
- add pr_debug() for the case when do_adjtimex() fails (Alex Ng)

Original description:

With TimeSync version 4 protocol support we started updating system time
continuously through the whole lifetime of Hyper-V guests. Every 5 seconds
there is a time sample from the host which triggers do_settimeofday[64]().
While the time from the host is very accurate such adjustments may cause
issues:
- Time is jumping forward and backward, some applications may misbehave.
- In case an NTP client is run in parallel things may go south, e.g. when
  an NTP client tries to adjust tick/frequency with ADJ_TICK/ADJ_FREQUENCY
  the Hyper-V module will not see this changes and time will oscillate and
  never converge.
- Systemd starts annoying you by printing "Time has been changed" every 5
  seconds to the system log.

With this series I suggest to use do_adjtimex() to adjust time. My tests
show that such method gives equally good time convergence but avoids all
the drawbacks described above.

Vitaly Kuznetsov (4):
  timekeeping: export do_adjtimex() to modules
  hv_util: switch to using timespec64
  hv_util: use do_adjtimex() to update system time
  hv_util: improve time adjustment accuracy by disabling interrupts

 drivers/hv/hv_util.c      | 40 ++++++++++++++++++++++++++++++++++++----
 kernel/time/timekeeping.c |  1 +
 2 files changed, 37 insertions(+), 4 deletions(-)

-- 
2.9.3