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When the first hrtimer on the current CPU is removed, hrtimer_force_reprogram() is invoked but only when CONFIG_HIGH_RES_TIMERS=y and hrtimer_cpu_base.hres_active is set. hrtimer_force_reprogram() updates hrtimer_cpu_base.expires_next and reprograms the clock event device. When CONFIG_HIGH_RES_TIMERS=y and hrtimer_cpu_base.hres_active is set, a pointless hrtimer interrupt can be prevented. hrtimer_check_target() makes the 'can remote enqueue' decision. As soon as hrtimer_check_target() is unconditionally available and hrtimer_cpu_base.expires_next is updated by hrtimer_reprogram(), hrtimer_force_reprogram() needs to be available unconditionally as well to prevent the following scenario with CONFIG_HIGH_RES_TIMERS=n: - the first hrtimer on this CPU is removed and hrtimer_force_reprogram() is not executed - CPU goes idle (next timer is calculated and hrtimers are taken into account) - a hrtimer is enqueued remote on the idle CPU: hrtimer_check_target() compares expiry value and hrtimer_cpu_base.expires_next. The expiry value is after expires_next, so the hrtimer is enqueued. This timer will fire late, if it expires before the effective first hrtimer on this CPU and the comparison was with an outdated expires_next value. To prevent this scenario, make hrtimer_force_reprogram() unconditional except the effective reprogramming part, which gets eliminated by the compiler in the CONFIG_HIGH_RES_TIMERS=n case. Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de> Cc: Christoph Hellwig <hch@lst.de> Cc: John Stultz <john.stultz@linaro.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: keescook@chromium.org Link: http://lkml.kernel.org/r/20171221104205.7269-20-anna-maria@linutronix.de Signed-off-by: Ingo Molnar <mingo@kernel.org>
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.