Messages in this thread | | | Date | Tue, 20 Aug 2019 17:20:25 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/core: Schedule new worker even if PI-blocked |
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On Tue, Aug 20, 2019 at 04:59:26PM +0200, Sebastian Andrzej Siewior wrote: > 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…
Well, yes, I too was/am sceptical, but it was tglx who twisted my arm and said the i2c people were right and rt_mutex is/should-be a generic usable interface.
This then resulted in the futex specific interface and lockdep support for rt_mutex:
5293c2efda37 ("futex,rt_mutex: Provide futex specific rt_mutex API") f5694788ad8d ("rt_mutex: Add lockdep annotations")
> > > 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.
All good, just trying to make sense of things :-)
> > > --- 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; > > > > > > /*
So this part actually makes rt_mutex less special and is good.
> > > @@ -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.
And am I right in thinking that that, again, is specific to the sleeping-spinlocks from PREEMPT_RT? Is there really nothing else that identifies those more specifically? It's been a while since I looked at them.
Also, I suppose it would be really good to put that in a comment.
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