Re: Query regarding synchronize_sched_expedited and resched_cpu

[Paul E. McKenney]

On Mon, Sep 18, 2017 at 12:12:13PM -0400, Steven Rostedt wrote:
> On Mon, 18 Sep 2017 09:01:25 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> 
> >     sched: Make resched_cpu() unconditional
> >     
> >     The current implementation of synchronize_sched_expedited() incorrectly
> >     assumes that resched_cpu() is unconditional, which it is not.  This means
> >     that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
> >     fails as follows (analysis by Neeraj Upadhyay):
> >     
> >     o    CPU1 is waiting for expedited wait to complete:
> >     sync_rcu_exp_select_cpus
> >          rdp->exp_dynticks_snap & 0x1   // returns 1 for CPU5
> >          IPI sent to CPU5
> >     
> >     synchronize_sched_expedited_wait
> >              ret = swait_event_timeout(
> >                                          rsp->expedited_wq,
> >       sync_rcu_preempt_exp_done(rnp_root),
> >                                          jiffies_stall);
> >     
> >                 expmask = 0x20 , and CPU 5 is in idle path (in cpuidle_enter())
> >     
> >     o    CPU5 handles IPI and fails to acquire rq lock.
> >     
> >     Handles IPI
> >          sync_sched_exp_handler
> >              resched_cpu
> >                  returns while failing to try lock acquire rq->lock
> >              need_resched is not set
> >     
> >     o    CPU5 calls  rcu_idle_enter() and as need_resched is not set, goes to
> >          idle (schedule() is not called).
> >     
> >     o    CPU 1 reports RCU stall.
> >     
> >     Given that resched_cpu() is used only by RCU, this commit fixes the
> >     assumption by making resched_cpu() unconditional.
> 
> Probably want to run this with several workloads with lockdep enabled
> first.

As soon as I work through the backlog of lockdep complaints that
appeared in the last merge window...  :-(

sparse_irq_lock, I am looking at you!!!  ;-)

							Thanx, Paul