Re: [PATCH] sched: Switch wait_task_inactive to HRTIMER_MODE_REL_HARD

[Peter Zijlstra]

On Thu, Aug 26, 2021 at 07:04:08PM +0200, Sebastian Andrzej Siewior wrote:
> With PREEMPT_RT enabled all hrtimers callbacks will be invoked in
> softirq mode unless they are explicitly marked as HRTIMER_MODE_HARD.
> During boot kthread_bind() is used for the creation of per-CPU threads
> and then hangs in wait_task_inactive() if the ksoftirqd is not
> yet up and running.
> The hang disappeared since commit
>    26c7295be0c5e ("kthread: Do not preempt current task if it is going to call schedule()")
> 
> but enabling function trace on boot reliably leads to the freeze on boot
> behaviour again.
> The timer in wait_task_inactive() can not be directly used by a user
> interface to abuse it and create a mass wake up of several tasks at the
> same time leading to long sections with disabled interrupts.
> Therefore it is safe to make the timer HRTIMER_MODE_REL_HARD.
> 
> Switch the timer to HRTIMER_MODE_REL_HARD.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

Thanks!