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Date:   Sat, 24 Sep 2022 00:13:49 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Pingfan Liu <kernelfans@gmail.com>, rcu@vger.kernel.org,
        David Woodhouse <dwmw@amazon.co.uk>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCHv2 3/3] rcu: coordinate tick dependency during concurrent
 offlining
Message-ID: <20220923221349.GA161211@lothringen>
References: <20220915055825.21525-1-kernelfans@gmail.com>
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 <20220920191339.GS4196@paulmck-ThinkPad-P17-Gen-1>
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On Thu, Sep 22, 2022 at 06:54:42AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 22, 2022 at 05:29:32PM +0800, Pingfan Liu wrote:
> > My understanding is after removing the tick dep by
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 79aea7df4345..cbfc884f04a4 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2171,8 +2171,6 @@ int rcutree_dead_cpu(unsigned int cpu)
> >         WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> >         /* Adjust any no-longer-needed kthreads. */
> >         rcu_boost_kthread_setaffinity(rnp, -1);
> > -       // Stop-machine done, so allow nohz_full to disable tick.
> > -       tick_dep_clear(TICK_DEP_BIT_RCU);
> >         return 0;
> >  }
> > 
> > @@ -4008,8 +4006,6 @@ int rcutree_online_cpu(unsigned int cpu)
> >         sync_sched_exp_online_cleanup(cpu);
> >         rcutree_affinity_setting(cpu, -1);
> > 
> > -       // Stop-machine done, so allow nohz_full to disable tick.
> > -       tick_dep_clear(TICK_DEP_BIT_RCU);
> >         return 0;
> >  }
> > 
> > @@ -4031,8 +4027,6 @@ int rcutree_offline_cpu(unsigned int cpu)
> > 
> >         rcutree_affinity_setting(cpu, cpu);
> > 
> > -       // nohz_full CPUs need the tick for stop-machine to work quickly
> > -       tick_dep_set(TICK_DEP_BIT_RCU);
> >         return 0;
> >  }
> > 
> > If the TREE04 can success, then move on to revert the commit(s)
> > identified by Frederic, and do test again.
> > 
> > At this time, a TREE04 failure is expected.
> > 
> > If the above two results are observed, TICK_DEP_BIT_RCU can be
> > removed.
> > 
> > Is my understanding right?
> 
> Seems plausible to me, but I again defer to Frederic.

Right, more precisely the commit to revert is:

     96926686deab ("rcu: Make CPU-hotplug removal operations enable tick")

which is the diff Pingfan Liu proposed above. The other uses of TICK_DEP_BIT_RCU
will still be needed though.

Thanks.