Re: [PATCH] rcu: Remove duplicate RCU exp QS report from rcu_report_dead()

[Paul E. McKenney]

On Tue, Aug 30, 2022 at 11:30:39AM +0000, Zhang, Qiang1 wrote:
> On Tue, Aug 30, 2022 at 04:31:51PM +0800, Zqiang wrote:
> > For PREEMPT_RCU, the rcu_report_dead() is invoked means that the
> > outgoing CPU mask is clear from leaf rcu_node and has no further
> > need of RCU, so invoke rcu_preempt_depth() return value is always
> > zero in rcu_report_dead(), if the current outgoing CPU rcu_data
> > structure's cpu_no_qs.b.exp is true, the rcu_preempt_deferred_qs()
> > will invoke rcu_report_exp_rdp() to report exp QS.
> > 
> > for non-PREEMPT_RCU, the rcu_preempt_deferred_qs() is equivalent to
> > rcu_report_exp_rdp().
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> >
> >Nice!  
> >
> >One question... Currently, for PREEMPT_RCU, the outgoing CPU silently
> >reports a quiescent state even if there was a bug that resulted in that
> >CPU still being in an RCU read-side critical section.  With your change,
> >the outgoing CPU would silently refuse to report a quiescent state.
> >
> >Is there something along the CPU-offline code path that already complains
> >about this situation?  If not, I believe that the first WARN_ON_ONCE()
> >in rcu_implicit_dynticks_qs() would complain.
> 
> In the following code, the current CPU will report QS if (rnp->qsmask & mask) return true.
> it means that the WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp)) is not trigger.
> 
> if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
>                 /* Report quiescent state -before- changing ->qsmaskinitnext! */
>                 rcu_disable_urgency_upon_qs(rdp);
>                 rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
>                 raw_spin_lock_irqsave_rcu_node(rnp, flags);
>         }
>  WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);
> 
> >
> >Could you please try this, just so we know what happens in this case?
> >One way of forcing this would be to do rcu_read_lock() just before the
> >call to rcu_report_dead(), though other diagnostics might require that
> >rcu_read_lock() to be earlier in the code.
> >
> >
> >Another question in both cases...  There is a more subtle change where the
> >old code ignores rdp->cpu_no_qs.b.exp (thus invoking rcu_report_exp_rdp()
> >unconditionally) and the new code avoids invoking rcu_report_exp_rdp()
> >unless this is set.  How does this interact with a new expedited
> >grace period that starts just as this CPU calls rcu_report_dead()?
> 
> 1.When a new expedited grace period that starts just as this CPU call rcu_report_dead(),
> if in this time, this CPU rcu_data structure's cpu_no_qs.b.exp is not set true, 
> the rcu_preempt_deferred_qs() will not call rcu_report_exp_rdp().  but when call
> rcu_report_dead(),  this CPU have been offline(cpu_is_offline(this CPU) return true).
> 
> 2.In __sync_rcu_exp_select_node_cpus(), invoke smp_call_function_single() for this
> CPU will return -ENXIO,  and then check  (rnp->qsmaskinitnext & mask) and (rnp->expmask & mask) 
> 
> 3.If in this time, the rcu_report_dead() has not yet clear CPU mask from rnp->qsmaskinitnext,
> we will redo 2 step,  recall smp_call_function_single(), but is always return -ENXIO, 
> and recheck (rnp->qsmaskinitnext & mask) and (rnp->expmask & mask), until rcu_report_dead()
> clear CPU mask from rnp->qsmaskinitnext.  
> 
> Therefore, the __sync_rcu_exp_select_node_cpus() will call rcu_report_exp_cpu_mult()
> to report this offline CPU's exp QS 

Very good, thank you!

Please see below for the newly applied patch with the usual wordsmithing.
As always, please let me know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit d51fbcdfb30dbe6e2e9c5874b9821f2c78092b2e
Author: Zqiang <qiang1.zhang@xxxxxxxxx>
Date:   Tue Aug 30 16:31:51 2022 +0800

    rcu: Remove duplicate RCU exp QS report from rcu_report_dead()
    
    The rcu_report_dead() function invokes rcu_report_exp_rdp() in order
    to force an immediate expedited quiescent state on the outgoing
    CPU, and then it invokes rcu_preempt_deferred_qs() to provide any
    required deferred quiescent state of either sort.  Because the call to
    rcu_preempt_deferred_qs() provides the expedited RCU quiescent state if
    requested, the call to rcu_report_exp_rdp() is potentially redundant.
    
    One possible issue is a concurrent start of a new expedited RCU
    grace period, but this situation is already handled correctly
    by __sync_rcu_exp_select_node_cpus().  This function will detect
    that the CPU is going offline via the error return from its call
    to smp_call_function_single().  In that case, it will retry, and
    eventually stop retrying due to rcu_report_exp_rdp() clearing the
    ->qsmaskinitnext bit corresponding to the target CPU.  As a result,
    __sync_rcu_exp_select_node_cpus() will report the necessary quiescent
    state after dealing with any remaining CPU.
    
    This change assumes that control does not enter rcu_report_dead() within
    an RCU read-side critical section, but then again, the surviving call
    to rcu_preempt_deferred_qs() has always made this assumption.
    
    This commit therefore removes the call to rcu_report_exp_rdp(), thus
    relying on rcu_preempt_deferred_qs() to handle both normal and expedited
    quiescent states.
    
    Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6bb8e72bc8151..0ca21ac0f0648 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4276,8 +4276,6 @@ void rcu_report_dead(unsigned int cpu)
 	// Do any dangling deferred wakeups.
 	do_nocb_deferred_wakeup(rdp);
 
-	/* QS for any half-done expedited grace period. */
-	rcu_report_exp_rdp(rdp);
 	rcu_preempt_deferred_qs(current);
 
 	/* Remove outgoing CPU from mask in the leaf rcu_node structure. */