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

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Date:   Wed, 7 Oct 2020 16:17:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, neeraju@...eaurora.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] rcu/tree: nocb: Avoid raising softirq when there are
 ready to execute CBs

On Thu, Oct 08, 2020 at 01:13:46AM +0200, Frederic Weisbecker wrote:
> 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@...eaurora.org
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > 
> > 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@...lfernandes.org>
> > 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@...eaurora.org>
> >     Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Applied, and thank you very much!

							Thanx, Paul