linux-kernel - Re: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head

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Message-ID: <20220608005513.GK1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 7 Jun 2022 17:55:13 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     "frederic@...nel.org" <frederic@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp
 list is empty

On Wed, Jun 08, 2022 at 12:41:28AM +0000, Zhang, Qiang1 wrote:
> On Tue, Jun 07, 2022 at 03:50:57PM +0800, Zqiang wrote:
> > Currently, If the 'rcu_nocb_poll' bootargs is enable, all rcuog 
> > kthreads enter polling mode. however, due to only insert CPU's rdp 
> > which belong to rcu_nocb_mask to 'nocb_head_rdp' list or all CPU's rdp 
> > served by rcuog kthread have been de-offloaded, these cause the 
> > 'nocb_head_rdp' list served by rcuog kthread is empty, when the 
> > 'nocb_head_rdp' is empty, the rcuog kthread in polling mode not 
> > actually do anything. fix it by exiting polling mode when the 
> > 'nocb_head_rdp'list is empty, otherwise entering polling mode.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> >
> >This looks a bit more plausible, but what have you done to test this?
> 
> Yes , I have only tested as follows and I added two trace events.
> 
> +                      trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EnterNoPoll"));
>                         rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
> +                       trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("ExitNoPoll"));
> 
> runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4" bootparams="rcu_nocbs=2,3 rcutree.dump_tree=1 rcu_nocb_poll rcutorture.nocbs_nthreads=4 rcutorture.test_boost=0" -d

To exercise your change, could you please also add an appropriate value
for the rcutorture.nocbs_nthreads kernel boot parameter?  Without that
boot parameter, your kernel will not actually offload or deoffload
any CPUs.

An alternative approach is to run rcutorture scenario TREE01.

							Thanx, Paul

> Thanks
> Zqiang
> 
> >
> >							Thanx, Paul
> >
> > ---
> >  v1->v2:
> >  Move rcu_nocb_poll flags check from rdp_offload_toggle() to  
> > rcu_nocb_rdp_offload/deoffload(), avoid unnecessary setting of  
> > rdp_gp->nocb_gp_sleep flags, because when rcu_nocb_poll is set  the 
> > rdp_gp->nocb_gp_sleep is not used.
> > 
> >  kernel/rcu/tree_nocb.h | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 
> > fa8e4f82e60c..2a52c9abc681 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -698,10 +698,14 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> >  				   TPS("WakeBypassIsDeferred"));
> >  	}
> >  	if (rcu_nocb_poll) {
> > -		/* Polling, so trace if first poll in the series. */
> > -		if (gotcbs)
> > -			trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> > -		schedule_timeout_idle(1);
> > +		if (list_empty(&my_rdp->nocb_head_rdp)) {
> > +			rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
> > +		} else {
> > +			/* Polling, so trace if first poll in the series. */
> > +			if (gotcbs)
> > +				trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> > +			schedule_timeout_idle(1);
> > +		}
> >  	} else if (!needwait_gp) {
> >  		/* Wait for callbacks to appear. */
> >  		trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep")); @@ -1030,7 
> > +1034,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
> >  
> >  	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
> >  	if (rdp_gp->nocb_gp_kthread) {
> > -		if (wake_gp)
> > +		if (wake_gp || rcu_nocb_poll)
> >  			wake_up_process(rdp_gp->nocb_gp_kthread);
> >  
> >  		/*
> > @@ -1152,7 +1156,7 @@ static long rcu_nocb_rdp_offload(void *arg)
> >  	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
> >  	 */
> >  	wake_gp = rdp_offload_toggle(rdp, true, flags);
> > -	if (wake_gp)
> > +	if (wake_gp || rcu_nocb_poll)
> >  		wake_up_process(rdp_gp->nocb_gp_kthread);
> >  	swait_event_exclusive(rdp->nocb_state_wq,
> >  			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
> > --
> > 2.25.1
> >