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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: Thomas 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.de Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.