• Thomas Gleixner's avatar
    timekeeping: Make the MONOTONIC clock behave like the BOOTTIME clock · d6ed449a
    Thomas Gleixner authored
    The MONOTONIC clock is not fast forwarded by the time spent in suspend on
    resume. This is only done for the BOOTTIME clock. The reason why the
    MONOTONIC clock is not forwarded is historical: the original Linux
    implementation was using jiffies as a base for the MONOTONIC clock and
    jiffies have never been advanced after resume.
    
    At some point when timekeeping was unified in the core code, the
    MONONOTIC clock was advanced after resume which also advanced jiffies causing
    interesting side effects. As a consequence the the MONOTONIC clock forwarding
    was disabled again and the BOOTTIME clock was introduced, which allows to read
    time since boot.
    
    Back then it was not possible to completely distangle the MONOTONIC clock and
    jiffies because there were still interfaces which exposed the MONOTONIC clock
    behaviour based on the timer wheel and therefore jiffies.
    
    As of today none of the MONOTONIC clock facilities depends on jiffies
    anymore so the forwarding can be done seperately. This is achieved by
    forwarding the variables which are used for the jiffies update after resume
    before the tick is restarted,
    
    In timekeeping resume, the change is rather simple. Instead of updating the
    offset between the MONOTONIC clock and the REALTIME/BOOTTIME clocks, advance the
    time keeper base for the MONOTONIC and the MONOTONIC_RAW clocks by the time
    spent in suspend.
    
    The MONOTONIC clock is now the same as the BOOTTIME clock and the offset between
    the REALTIME and the MONOTONIC clocks is the same as before suspend.
    
    There might be side effects in applications, which rely on the
    (unfortunately) well documented behaviour of the MONOTONIC clock, but the
    downsides of the existing behaviour are probably worse.
    
    There is one obvious issue. Up to now it was possible to retrieve the time
    spent in suspend by observing the delta between the MONOTONIC clock and the
    BOOTTIME clock. This is not longer available, but the previously introduced
    mechanism to read the active non-suspended monotonic time can mitigate that
    in a detectable fashion.
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
    Cc: John Stultz <john.stultz@linaro.org>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Kevin Easton <kevin@guarana.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Mark Salyzyn <salyzyn@android.com>
    Cc: Michael Kerrisk <mtk.manpages@gmail.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Petr Mladek <pmladek@suse.com>
    Cc: Prarit Bhargava <prarit@redhat.com>
    Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Link: http://lkml.kernel.org/r/20180301165150.062975504@linutronix.deSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    d6ed449a
tick-common.c 13.8 KB