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For a long time the tick was aligned to clock MONOTONIC so that the tick event happened at a multiple of nanoseconds per tick starting from clock MONOTONIC = 0. At some point this changed as the refined jiffies clocksource which is used during boot before the TSC or other clocksources becomes usable, was adjusted with a boot offset, so that time 0 is closer to the point where the kernel starts. This broke the assumption in the tick code that when the tick setup happens early on ktime_get() will return a multiple of nanoseconds per tick. As a consequence applications which aligned their periodic execution so that it does not collide with the tick were not longer guaranteed that the tick period starts from time 0. The fix for this regression was to realign the tick when it is initially set up to a multiple of tick periods. That works as long as the underlying tick device supports periodic mode, but breaks under certain conditions when the tick device supports only one shot mode. Depending on the offset, the alignment delta to clock MONOTONIC can get in a range where the minimal programming delta of the underlying clock event device is larger than the calculated delta to the next tick. This results in a boot hang as the tick code tries to play catch up, but as the tick never fires jiffies are not advanced so it keeps trying for ever. Solve this by moving the tick alignement into the NOHZ / HIGHRES enablement code because at that point it is guaranteed that the underlying clocksource is high resolution capable and not longer depending on the tick. This is far before user space starts, so at the point where applications try to align their timers, the old behaviour of the tick happening at a multiple of nanoseconds per tick starting from clock MONOTONIC = 0 is restored. -----BEGIN PGP SIGNATURE----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAmSTUNQTHHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYoQQjD/4+zHK7IxRK+1k1lIStxksEudVeNuqK 7RZKNTawZFSChrxGmv5uhmqYDK9/tudnZot+uyAXI81JHnnGSXbco+VrAA2DQ2v2 w/oxhAHIAobJWDzUuCqT2YsseDVNoTGMrTTPrL4klXqhVtShVOLTI/603+macEiw Olqmd7XxlyhCloNqoMIh18EwSNIs78kP3YAQHUmq3NoRkJT5c3wWKvZy38WOxSJ9 Jjpw75tePfcwWte9fGVq+oJ8hOWkPAKN5hFL8420RrEgdIkxKlhVmuyG48ieZKH1 LE1brB5iaU8+M91PevZMRlD7oio8u/d6xX1vT3Cqz9UBe1NLj1I0ToX6COhwSNxm 50U0HJ5wGlpodu4IyBHXTG45UAVwpOg/EgDlFigeP3Nxl6h45ugrA010r6FqE1zV KltjNGsk/OD5d8ywW+yManU2mySm9znqmWN2zbjRGm138Tuz7vglcBhFB6jEJFUF V8vYPLwI4J2gqifx8NoFBIfLuZp/+D4LOQiJ7XRc9CVm3JLK8Xi8fxnHGTBwJHIn 45WU4hzmY7fjR/gGhAyG7lH6tlo7VTC9/lrO+YtAy2hL2sK14+m/ZH7836I5bAhw EgSyVlcfR6tgO55I3JgkbNIyO5TUAZPZIAY1TlyR53xXvMkb9aZTh9UMm1xMJ3uX +sQaLAuDFNUQSg== =/uvx -----END PGP SIGNATURE----- Merge tag 'timers-urgent-2023-06-21' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull timer fix from Thomas Gleixner: "A single regression fix for a regression fix: For a long time the tick was aligned to clock MONOTONIC so that the tick event happened at a multiple of nanoseconds per tick starting from clock MONOTONIC = 0. At some point this changed as the refined jiffies clocksource which is used during boot before the TSC or other clocksources becomes usable, was adjusted with a boot offset, so that time 0 is closer to the point where the kernel starts. This broke the assumption in the tick code that when the tick setup happens early on ktime_get() will return a multiple of nanoseconds per tick. As a consequence applications which aligned their periodic execution so that it does not collide with the tick were not longer guaranteed that the tick period starts from time 0. The fix for this regression was to realign the tick when it is initially set up to a multiple of tick periods. That works as long as the underlying tick device supports periodic mode, but breaks under certain conditions when the tick device supports only one shot mode. Depending on the offset, the alignment delta to clock MONOTONIC can get in a range where the minimal programming delta of the underlying clock event device is larger than the calculated delta to the next tick. This results in a boot hang as the tick code tries to play catch up, but as the tick never fires jiffies are not advanced so it keeps trying for ever. Solve this by moving the tick alignement into the NOHZ / HIGHRES enablement code because at that point it is guaranteed that the underlying clocksource is high resolution capable and not longer depending on the tick. This is far before user space starts, so at the point where applications try to align their timers, the old behaviour of the tick happening at a multiple of nanoseconds per tick starting from clock MONOTONIC = 0 is restored" * tag 'timers-urgent-2023-06-21' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: tick/common: Align tick period during sched_timer setup |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
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.