Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | From | Yu Liao <> | Subject | [PATCH] tick/broadcast-hrtimer: Prevent the timer device on broadcast duty CPU from being disabled | Date | Mon, 18 Dec 2023 10:58:44 +0800 |
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It was found that running the LTP hotplug stress test on a aarch64 system could produce rcu_sched stall warnings.
The issue is the following:
CPU1 (owns the broadcast hrtimer) CPU2
tick_broadcast_enter() //shut down local timer device ... tick_broadcast_exit() //exits with tick_broadcast_force_mask set, timer device remains disabled
initiates offlining of CPU1 take_cpu_down() //CPU1 shuts down and does not send broadcast IPI anymore takedown_cpu() hotplug_cpu__broadcast_tick_pull() //move broadcast hrtimer to this CPU clockevents_program_event() bc_set_next() hrtimer_start() //does not call hrtimer_reprogram() to program timer device if expires equals dev->next_event, so the timer device remains disabled.
CPU2 takes over the broadcast duty but local timer device is disabled, causing many CPUs to become stuck.
Fix this by calling tick_program_event() to reprogram the local timer device in this scenario.
Signed-off-by: Yu Liao <liaoyu15@huawei.com> --- kernel/time/tick-broadcast-hrtimer.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c index e28f9210f8a1..6a4a612581fb 100644 --- a/kernel/time/tick-broadcast-hrtimer.c +++ b/kernel/time/tick-broadcast-hrtimer.c @@ -42,10 +42,22 @@ static int bc_shutdown(struct clock_event_device *evt) */ static int bc_set_next(ktime_t expires, struct clock_event_device *bc) { + ktime_t next_event = this_cpu_ptr(&tick_cpu_device)->evtdev->next_event; + /* - * This is called either from enter/exit idle code or from the - * broadcast handler. In all cases tick_broadcast_lock is held. - * + * This can be called from CPU offline operation to move broadcast + * assignment. If tick_broadcast_force_mask is set, the CPU local + * timer device may be disabled. And hrtimer_reprogram() will not + * called if the timer is not the first expiring timer. Reprogram + * the cpu local timer device to ensure we can take over the + * broadcast duty. + */ + if (tick_check_broadcast_expired() && expires >= next_event) + tick_program_event(next_event, 1); + + /* + * This is called from enter/exit idle code, broadcast handler or + * CPU offline operation. In all cases tick_broadcast_lock is held. * hrtimer_cancel() cannot be called here neither from the * broadcast handler nor from the enter/exit idle code. The idle * code can run into the problem described in bc_shutdown() and the -- 2.33.0
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