Re: [PATCH v2] rcu/tree: nocb: Avoid raising softirq when there are ready to execute CBs

[Frederic Weisbecker]

On Wed, Oct 07, 2020 at 03:34:38PM -0700, Paul E. McKenney wrote:
> On Sun, Oct 04, 2020 at 10:11:32PM -0400, Joel Fernandes (Google) wrote:
> > During testing, I see it is possible that rcu_pending() returns 1 when
> > offloaded callbacks are ready to execute thus raising the RCU softirq.
> > 
> > However, softirq does not execute offloaded callbacks. They are executed in a
> > kthread which is awakened independent of the softirq.
> > 
> > This commit therefore avoids raising the softirq in the first place. That's
> > probably a good thing considering that the purpose of callback offloading is to
> > reduce softirq activity.
> > 
> > Passed 30 minute tests of TREE01 through TREE09 each.
> > 
> > On TREE08, I notice that there is atmost 150us from when the softirq was
> > NOT raised when ready cbs were present, to when the ready callbacks were
> > invoked by the rcuop thread. This also further confirms that there is no
> > need to raise the softirq for ready cbs in the first place.
> > 
> > Cc: neeraju@xxxxxxxxxxxxxx
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> 
> Looks good, applied, thank you!  I reworked things a bit based on
> previous patches and to more precisely capture why this patch does
> not cause additional problems.  Please let me know if I messed
> anything up.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 33847a34a2d261354a79b4a24d9d37222e8ec888
> Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Date:   Wed Oct 7 13:50:36 2020 -0700
> 
>     rcu/tree: nocb: Avoid raising softirq for offloaded ready-to-execute CBs
>     
>     Testing showed that rcu_pending() can return 1 when offloaded callbacks
>     are ready to execute.  This invokes RCU core processing, for example,
>     by raising RCU_SOFTIRQ, eventually resulting in a call to rcu_core().
>     However, rcu_core() explicitly avoids in any way manipulating offloaded
>     callbacks, which are instead handled by the rcuog and rcuoc kthreads,
>     which work independently of rcu_core().
>     
>     One exception to this independence is that rcu_core() invokes
>     do_nocb_deferred_wakeup(), however, rcu_pending() also checks
>     rcu_nocb_need_deferred_wakeup() in order to correctly handle this case,
>     invoking rcu_core() when needed.
>     
>     This commit therefore avoids needlessly invoking RCU core processing
>     by checking rcu_segcblist_ready_cbs() only on non-offloaded CPUs.
>     This reduces overhead, for example, by reducing softirq activity.
>     
>     This change passed 30 minute tests of TREE01 through TREE09 each.
>     
>     On TREE08, there is at most 150us from the time that rcu_pending() chose
>     not to invoke RCU core processing to the time when the ready callbacks
>     were invoked by the rcuoc kthread.  This provides further evidence that
>     there is no need to invoke rcu_core() for offloaded callbacks that are
>     ready to invoke.
>     
>     Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
>     Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Thanks!