On 2019-08-20 15:50:14 [+0200], Peter Zijlstra wrote: > On Fri, Aug 16, 2019 at 06:06:26PM +0200, Sebastian Andrzej Siewior wrote: > > If a task is PI-blocked (blocking on sleeping spinlock) then we don't want > > to > > schedule a new kworker if we schedule out due to lock contention because !RT > > does not do that as well. > > s/as well/either/ > > > A spinning spinlock disables preemption and a worker > > does not schedule out on lock contention (but spin). > > I'm not much liking this; it means that rt_mutex and mutex have > different behaviour, and there are 'normal' rt_mutex users in the tree.
There isc RCU (boosting) and futex. I'm sceptical about the i2c users… > > On RT the RW-semaphore implementation uses an rtmutex so > > tsk_is_pi_blocked() will return true if a task blocks on it. In this case we > > will now start a new worker > > I'm confused, by bailing out early it does _NOT_ start a new worker; or > am I reading it wrong? s@now@not@. Your eyes work good, soory for that. > > which may deadlock if one worker is waiting on > > progress from another worker. > > > Since a RW-semaphore starts a new worker on !RT, we should do the same on > > RT. > > > > XFS is able to trigger this deadlock. > > > > Allow to schedule new worker if the current worker is PI-blocked. > > Which contradicts earlier parts of this changelog. > > > Signed-off-by: Sebastian Andrzej Siewior <[email protected]> > > --- > > kernel/sched/core.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -3945,7 +3945,7 @@ void __noreturn do_task_dead(void) > > > > static inline void sched_submit_work(struct task_struct *tsk) > > { > > - if (!tsk->state || tsk_is_pi_blocked(tsk)) > > + if (!tsk->state) > > return; > > > > /* > > @@ -3961,6 +3961,9 @@ static inline void sched_submit_work(str > > preempt_enable_no_resched(); > > } > > > > + if (tsk_is_pi_blocked(tsk)) > > + return; > > + > > /* > > * If we are going to sleep and we have plugged IO queued, > > * make sure to submit it to avoid deadlocks. > > What do we need that clause for? Why is pi_blocked special _at_all_? so !RT the scheduler does nothing special if a task blocks on sleeping lock. If I remember correctly then blk_schedule_flush_plug() is the problem. It may require a lock which is held by the task. It may hold A and wait for B while another task has B and waits for A. If my memory does bot betray me then ext+jbd can lockup without this. Sebastian
