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Date: Mon, 14 Dec 2020 12:06:24 +0700 From: Pavel <biohumanoid.lists@...il.com> To: linux-kernel@...r.kernel.org Subject: Bug in recent kernel timer implementation Hi, Seems like kernel timer implementation is buggy since some kernel version (between 3.16.0 and 4.9.0). Comments for add_timer in timer.c: * The kernel will do a ->function(@timer) callback from the * timer interrupt at the ->expires point in the future. The * current time is 'jiffies'. Linux x86, Linux x64. HZ=250. init_timer(&my_timer); my_timer.function = testjiffy_timer_function; my_timer.expires = jiffies + 1; add_timer(&my_timer); So, according to comments in add_timer, testjiffy_timer_function should be called next timer tick, 4ms later. If you will run this code in Linux 3.16.0 - it will call testjiffy_timer_function in the next timer tick (4ms later), as expected. If you will run this code in Linux 4.9.0 - it will call testjiffy_timer_function in the on second timer tick (8ms later)... If you will modify code for Linux > 4.15 (setup_timer/mod_timer) - you will get the same strange behaviour... You can get code from here https://stackoverflow.com/questions/16920238/reliability-of-linux-kernel-add-timer-at-resolution-of-one-jiffy 3.16.0 [52728.313029] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4 [52728.313720] testjiffy_timer_function: runcount 1, jif=13110976, oldjif=13110975, delta=1 [52728.318743] testjiffy_timer_function: runcount 2, jif=13110977, oldjif=13110976, delta=1 [52728.323304] testjiffy_timer_function: runcount 3, jif=13110978, oldjif=13110977, delta=1 [52728.325824] testjiffy_timer_function: runcount 4, jif=13110979, oldjif=13110978, delta=1 [52728.330811] testjiffy_timer_function: runcount 5, jif=13110980, oldjif=13110979, delta=1 [52728.335802] testjiffy_timer_function: runcount 6, jif=13110981, oldjif=13110980, delta=1 [52728.338305] testjiffy_timer_function: runcount 7, jif=13110982, oldjif=13110981, delta=1 [52728.343298] testjiffy_timer_function: runcount 8, jif=13110983, oldjif=13110982, delta=1 [52728.345806] testjiffy_timer_function: runcount 9, jif=13110984, oldjif=13110983, delta=1 [52728.350885] testjiffy_timer_function: runcount 10, jif=13110985, oldjif=13110984, delta=1 [52729.317263] Exit testjiffy 4.9.0 [52073.085341] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4 [52073.092322] testjiffy_timer_function: runcount 1, jif=4307910976, oldjif=4307910974, delta=2 [52073.099818] testjiffy_timer_function: runcount 2, jif=4307910978, oldjif=4307910976, delta=2 [52073.109830] testjiffy_timer_function: runcount 3, jif=4307910980, oldjif=4307910978, delta=2 [52073.117362] testjiffy_timer_function: runcount 4, jif=4307910982, oldjif=4307910980, delta=2 [52073.124845] testjiffy_timer_function: runcount 5, jif=4307910984, oldjif=4307910982, delta=2 [52073.132315] testjiffy_timer_function: runcount 6, jif=4307910986, oldjif=4307910984, delta=2 [52073.139816] testjiffy_timer_function: runcount 7, jif=4307910988, oldjif=4307910986, delta=2 [52073.149824] testjiffy_timer_function: runcount 8, jif=4307910990, oldjif=4307910988, delta=2 [52073.157303] testjiffy_timer_function: runcount 9, jif=4307910992, oldjif=4307910990, delta=2 [52073.164824] testjiffy_timer_function: runcount 10, jif=4307910994, oldjif=4307910992, delta=2 [52074.089574] Exit testjiffy
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