linux/kernel/time, branch v6.8

hrtimer: Report offline hrtimer enqueue

2024-02-06T09:56:35Z

The hrtimers migration on CPU-down hotplug process has been moved earlier, before the CPU actually goes to die. This leaves a small window of opportunity to queue an hrtimer in a blind spot, leaving it ignored. For example a practical case has been reported with RCU waking up a SCHED_FIFO task right before the CPUHP_AP_IDLE_DEAD stage, queuing that way a sched/rt timer to the local offline CPU. Make sure such situations never go unnoticed and warn when that happens. Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") Reported-by: Paul E. McKenney Signed-off-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240129235646.3171983-4-boqun.feng@gmail.com

tick/sched: Preserve number of idle sleeps across CPU hotplug events

2024-01-25T08:52:40Z

Commit 71fee48f ("tick-sched: Fix idle and iowait sleeptime accounting vs CPU hotplug") preserved total idle sleep time and iowait sleeptime across CPU hotplug events. Similar reasoning applies to the number of idle calls and idle sleeps to get the proper average of sleep time per idle invocation. Preserve those fields too. Fixes: 71fee48f ("tick-sched: Fix idle and iowait sleeptime accounting vs CPU hotplug") Signed-off-by: Tim Chen Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240122233534.3094238-1-tim.c.chen@linux.intel.com

clocksource: Skip watchdog check for large watchdog intervals

2024-01-25T08:13:16Z

There have been reports of the watchdog marking clocksources unstable on machines with 8 NUMA nodes: clocksource: timekeeping watchdog on CPU373: Marking clocksource 'tsc' as unstable because the skew is too large: clocksource: 'hpet' wd_nsec: 14523447520 clocksource: 'tsc' cs_nsec: 14524115132 The measured clocksource skew - the absolute difference between cs_nsec and wd_nsec - was 668 microseconds: cs_nsec - wd_nsec = 14524115132 - 14523447520 = 667612 The kernel used 200 microseconds for the uncertainty_margin of both the clocksource and watchdog, resulting in a threshold of 400 microseconds (the md variable). Both the cs_nsec and the wd_nsec value indicate that the readout interval was circa 14.5 seconds. The observed behaviour is that watchdog checks failed for large readout intervals on 8 NUMA node machines. This indicates that the size of the skew was directly proportinal to the length of the readout interval on those machines. The measured clocksource skew, 668 microseconds, was evaluated against a threshold (the md variable) that is suited for readout intervals of roughly WATCHDOG_INTERVAL, i.e. HZ >> 1, which is 0.5 second. The intention of 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold") was to tighten the threshold for evaluating skew and set the lower bound for the uncertainty_margin of clocksources to twice WATCHDOG_MAX_SKEW. Later in c37e85c135ce ("clocksource: Loosen clocksource watchdog constraints"), the WATCHDOG_MAX_SKEW constant was increased to 125 microseconds to fit the limit of NTP, which is able to use a clocksource that suffers from up to 500 microseconds of skew per second. Both the TSC and the HPET use default uncertainty_margin. When the readout interval gets stretched the default uncertainty_margin is no longer a suitable lower bound for evaluating skew - it imposes a limit that is far stricter than the skew with which NTP can deal. The root causes of the skew being directly proportinal to the length of the readout interval are: * the inaccuracy of the shift/mult pairs of clocksources and the watchdog * the conversion to nanoseconds is imprecise for large readout intervals Prevent this by skipping the current watchdog check if the readout interval exceeds 2 * WATCHDOG_INTERVAL. Considering the maximum readout interval of 2 * WATCHDOG_INTERVAL, the current default uncertainty margin (of the TSC and HPET) corresponds to a limit on clocksource skew of 250 ppm (microseconds of skew per second). To keep the limit imposed by NTP (500 microseconds of skew per second) for all possible readout intervals, the margins would have to be scaled so that the threshold value is proportional to the length of the actual readout interval. As for why the readout interval may get stretched: Since the watchdog is executed in softirq context the expiration of the watchdog timer can get severely delayed on account of a ksoftirqd thread not getting to run in a timely manner. Surely, a system with such belated softirq execution is not working well and the scheduling issue should be looked into but the clocksource watchdog should be able to deal with it accordingly. Fixes: 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold") Suggested-by: Feng Tang Signed-off-by: Jiri Wiesner Signed-off-by: Thomas Gleixner Tested-by: Paul E. McKenney Reviewed-by: Feng Tang Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20240122172350.GA740@incl

Merge tag 'timers-core-2024-01-21' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2024-01-21T19:14:40Z

Pull timer updates from Thomas Gleixner: "Updates for time and clocksources: - A fix for the idle and iowait time accounting vs CPU hotplug. The time is reset on CPU hotplug which makes the accumulated systemwide time jump backwards. - Assorted fixes and improvements for clocksource/event drivers" * tag 'timers-core-2024-01-21' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: tick-sched: Fix idle and iowait sleeptime accounting vs CPU hotplug clocksource/drivers/ep93xx: Fix error handling during probe clocksource/drivers/cadence-ttc: Fix some kernel-doc warnings clocksource/drivers/timer-ti-dm: Fix make W=n kerneldoc warnings clocksource/timer-riscv: Add riscv_clock_shutdown callback dt-bindings: timer: Add StarFive JH8100 clint dt-bindings: timer: thead,c900-aclint-mtimer: separate mtime and mtimecmp regs

tick-sched: Fix idle and iowait sleeptime accounting vs CPU hotplug

2024-01-19T15:40:38Z

When offlining and onlining CPUs the overall reported idle and iowait times as reported by /proc/stat jump backward and forward: cpu 132 0 176 225249 47 6 6 21 0 0 cpu0 80 0 115 112575 33 3 4 18 0 0 cpu1 52 0 60 112673 13 3 1 2 0 0 cpu 133 0 177 226681 47 6 6 21 0 0 cpu0 80 0 116 113387 33 3 4 18 0 0 cpu 133 0 178 114431 33 6 6 21 0 0 <---- jump backward cpu0 80 0 116 114247 33 3 4 18 0 0 cpu1 52 0 61 183 0 3 1 2 0 0 <---- idle + iowait start with 0 cpu 133 0 178 228956 47 6 6 21 0 0 <---- jump forward cpu0 81 0 117 114929 33 3 4 18 0 0 Reason for this is that get_idle_time() in fs/proc/stat.c has different sources for both values depending on if a CPU is online or offline: - if a CPU is online the values may be taken from its per cpu tick_cpu_sched structure - if a CPU is offline the values are taken from its per cpu cpustat structure The problem is that the per cpu tick_cpu_sched structure is set to zero on CPU offline. See tick_cancel_sched_timer() in kernel/time/tick-sched.c. Therefore when a CPU is brought offline and online afterwards both its idle and iowait sleeptime will be zero, causing a jump backward in total system idle and iowait sleeptime. In a similar way if a CPU is then brought offline again the total idle and iowait sleeptimes will jump forward. It looks like this behavior was introduced with commit 4b0c0f294f60 ("tick: Cleanup NOHZ per cpu data on cpu down"). This was only noticed now on s390, since we switched to generic idle time reporting with commit be76ea614460 ("s390/idle: remove arch_cpu_idle_time() and corresponding code"). Fix this by preserving the values of idle_sleeptime and iowait_sleeptime members of the per-cpu tick_sched structure on CPU hotplug. Fixes: 4b0c0f294f60 ("tick: Cleanup NOHZ per cpu data on cpu down") Reported-by: Gerald Schaefer Signed-off-by: Heiko Carstens Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20240115163555.1004144-1-hca@linux.ibm.com

Merge tag 'timers-core-2024-01-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2024-01-09T02:44:11Z

Pull timer subsystem updates from Ingo Molnar: - Various preparatory cleanups & enhancements of the timer-wheel code, in preparation for the WIP 'pull timers at expiry' timer migration model series (which will replace the current 'push timers at enqueue' migration model), by Anna-Maria Behnsen: - Update comments and clean up confusing variable names - Add debug check to warn about time travel - Improve/expand timer-wheel tracepoints - Optimize away unnecessary IPIs for deferrable timers - Restructure & clean up next_expiry_recalc() - Clean up forward_timer_base() - Introduce __forward_timer_base() and use it to simplify and micro-optimize get_next_timer_interrupt() - Restructure the get_next_timer_interrupt()'s idle logic for better readability and to enable a minor optimization. - Fix the nextevt calculation when no timers are pending - Fix the sysfs_get_uname() prototype declaration * tag 'timers-core-2024-01-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: timers: Fix nextevt calculation when no timers are pending timers: Rework idle logic timers: Use already existing function for forwarding timer base timers: Split out forward timer base functionality timers: Clarify check in forward_timer_base() timers: Move store of next event into __next_timer_interrupt() timers: Do not IPI for deferrable timers tracing/timers: Add tracepoint for tracking timer base is_idle flag tracing/timers: Enhance timer_start tracepoint tick-sched: Warn when next tick seems to be in the past tick/sched: Cleanup confusing variables tick-sched: Fix function names in comments time: Make sysfs_get_uname() function visible in header

posix-timers: Get rid of [COMPAT_]SYS_NI() uses

2023-12-21T05:30:27Z

Only the posix timer system calls use this (when the posix timer support is disabled, which does not actually happen in any normal case), because they had debug code to print out a warning about missing system calls. Get rid of that special case, and just use the standard COND_SYSCALL interface that creates weak system call stubs that return -ENOSYS for when the system call does not exist. This fixes a kCFI issue with the SYS_NI() hackery: CFI failure at int80_emulation+0x67/0xb0 (target: sys_ni_posix_timers+0x0/0x70; expected type: 0xb02b34d9) WARNING: CPU: 0 PID: 48 at int80_emulation+0x67/0xb0 Reported-by: kernel test robot Reviewed-by: Sami Tolvanen Tested-by: Sami Tolvanen Cc: Thomas Gleixner Cc: Dave Hansen Cc: Borislav Petkov Signed-off-by: Linus Torvalds

timers: Fix nextevt calculation when no timers are pending

2023-12-20T15:49:39Z

When no timer is queued into an empty timer base, the next_expiry will not be updated. It was originally calculated as base->clk + NEXT_TIMER_MAX_DELTA When the timer base stays empty long enough (> NEXT_TIMER_MAX_DELTA), the next_expiry value of the empty base suggests that there is a timer pending soon. This might be more a kind of a theoretical problem, but the fix doesn't hurt. Use only base->next_expiry value as nextevt when timers are pending. Otherwise nextevt will be jiffies + NEXT_TIMER_MAX_DELTA. As all information is in place, update base->next_expiry value of the empty timer base as well. Signed-off-by: Anna-Maria Behnsen Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20231201092654.34614-13-anna-maria@linutronix.de

timers: Rework idle logic

2023-12-20T15:49:39Z

To improve readability of the code, split base->idle calculation and expires calculation into separate parts. While at it, update the comment about timer base idle marking. Thereby the following subtle change happens if the next event is just one jiffy ahead and the tick was already stopped: Originally base->is_idle remains true in this situation. Now base->is_idle turns to false. This may spare an IPI if a timer is enqueued remotely to an idle CPU that is going to tick on the next jiffy. Signed-off-by: Thomas Gleixner Signed-off-by: Anna-Maria Behnsen Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20231201092654.34614-12-anna-maria@linutronix.de

timers: Use already existing function for forwarding timer base

2023-12-20T15:49:38Z

There is an already existing function for forwarding the timer base. Forwarding the timer base is implemented directly in get_next_timer_interrupt() as well. Remove the code duplication and invoke __forward_timer_base() instead. Signed-off-by: Anna-Maria Behnsen Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20231201092654.34614-11-anna-maria@linutronix.de