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Setting DEC to maximum at the start of the timer interrupt is not necessary and can be avoided for performance when MSR[EE] is not enabled during the handler as explained in commit0faf20a1ad("powerpc/64s/interrupt: Don't enable MSR[EE] in irq handlers unless perf is in use"), where this change was first attempted. The idea is that the timer interrupt runs with MSR[EE]=0, and at the end of the interrupt DEC is programmed to the next timer interval, so there is no need to clear the decrementer exception before then. When the above commit was merged, that was not quite true. The low res timer subsystem had some cases in the oneshot timer code where if the tick was to be stopped and no timers active, the clock device would not get the ->set_state_oneshot_stopped() call, so DEC would not get reprogrammed, and this would hang taking continual timer interrupts. So this was reverted in commitd2b9be1f4a("powerpc/time: Always set decrementer in timer_interrupt()"), which was a partial revert of the above commit. Commit62c1256d54("timers/nohz: Switch to ONESHOT_STOPPED in the low-res handler when the tick is stopped") was later merged to fix this missing case in the timer subsystem, so now the behaviour can be restored. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20220909142457.278032-1-npiggin@gmail.com
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.