linux-kernel - Re: [PATCH] rcu/nocb: Clear rdp offloaded flags when rcuop/rcuog kthreads spawn failed

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Date:   Wed, 9 Mar 2022 22:06:57 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     "paulmck@...nel.org" <paulmck@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] rcu/nocb: Clear rdp offloaded flags when rcuop/rcuog
 kthreads spawn failed

On Tue, Mar 08, 2022 at 07:37:24AM +0000, Zhang, Qiang1 wrote:
> 
> On Mon, Feb 28, 2022 at 05:36:29PM +0800, Zqiang wrote:
> > When CONFIG_RCU_NOCB_CPU is enabled and 'rcu_nocbs' is set, the rcuop 
> > and rcuog kthreads is created. however the rcuop or rcuog kthreads 
> > creation may fail, if failed, clear rdp offloaded flags.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> > ---
> >  kernel/rcu/tree_nocb.h | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 
> > 46694e13398a..94b279147954 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1246,7 +1246,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
> >  				"rcuog/%d", rdp_gp->cpu);
> >  		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__)) {
> >  			mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
> > -			return;
> > +			goto end;
> >  		}
> >  		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
> >  		if (kthread_prio)
> > @@ -1258,12 +1258,22 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
> >  	t = kthread_run(rcu_nocb_cb_kthread, rdp,
> >  			"rcuo%c/%d", rcu_state.abbr, cpu);
> >  	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
> > -		return;
> > +		goto end;
> >  
> >  	if (kthread_prio)
> >  		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> >  	WRITE_ONCE(rdp->nocb_cb_kthread, t);
> >  	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
> > +	return;
> > +end:
> > +	if (cpumask_test_cpu(cpu, rcu_nocb_mask)) {
> > +		rcu_segcblist_offload(&rdp->cblist, false);
> > +		rcu_segcblist_clear_flags(&rdp->cblist,
> > +				SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
> > +		rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_LOCKING);
> > +		rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_RCU_CORE);
> > +	}
> >>
> >>Thanks you, consequences are indeed bad otherwise because the target is considered offloaded but nothing actually handles the callbacks.
> >>
> >>A few issues though:
> >>
> >>* The rdp_gp kthread may be running concurrently. If it's iterating this rdp and
> >>  the SEGCBLIST_LOCKING flag is cleared in the middle, rcu_nocb_unlock() won't
> >>  release (among many other possible issues).
> >>
> >>* we should clear the cpu from rcu_nocb_mask or we won't be able to later
> >>  re-offload it.
> >>
> >>* we should then delete the rdp from the group list:
> >>
> >>     list_del_rcu(&rdp->nocb_entry_rdp);
> >>
> >>So ideally we should call rcu_nocb_rdp_deoffload(). But then bear in mind:
> >>
> >>1) We must lock rcu_state.barrier_mutex and hotplug read lock. But since we
> >>   are calling rcutree_prepare_cpu(), we maybe holding hotplug write lock
> >>   already.
> >>
> >>   Therefore we first need to invert the locking dependency order between
> >>   rcu_state.barrier_mutex and hotplug lock and then just lock the barrier_mutex
> >>   before calling rcu_nocb_rdp_deoffload() from our failure path.
> >>   
> >>
> >>2) On rcu_nocb_rdp_deoffload(), handle non-existing nocb_gp and/or nocb_cb
> >>   kthreads. Make sure we are holding nocb_gp_kthread_mutex.
> 
> Sorry for my late reply,  Is the nocb_gp_kthread_mutex really necessary?
> Because the cpu online/offline is serial operation,  It is protected by  cpus_write_lock()

And you're right! But some people are working on making cpu_up() able to work
in parallel for faster bring-up on boot.