linux-kernel - Re: [PATCH] rcu/tree: Force quiescent state on callback overload

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Date:   Sun, 21 Jun 2020 20:13:25 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/tree: Force quiescent state on callback overload

On Mon, Jun 22, 2020 at 01:30:31AM +0530, Neeraj Upadhyay wrote:
> Hi Paul,
> 
> On 6/22/2020 1:20 AM, Paul E. McKenney wrote:
> > On Mon, Jun 22, 2020 at 12:07:27AM +0530, Neeraj Upadhyay wrote:
> > > On callback overload, we want to force quiescent state immediately,
> > > for the first and second fqs. Enforce the same, by including
> > > RCU_GP_FLAG_OVLD flag, in fqsstart check.
> > > 
> > > Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
> > 
> > Good catch!
> > 
> > But what did you do to verify that this change does the right thing?
> > 
> > 						Thanx, Paul
> > 
> 
> I haven't done a runtime verification of this code path; I posted this,
> based on review of this code.

My concern is that under overload, the FQS scans would happen continuously
rather than accelerating only the first such scan in a given grace period.
This would of course result in a CPU-bound grace-period kthread, which
users might not be all that happy with.

Or am I missing something subtle that prevents this?

But yes, it does look like the current mainline code fails to do the
first scan immediately, so again, good catch!

							Thanx, Paul

> Thanks
> Neeraj
> 
> > > ---
> > >   kernel/rcu/tree.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index d0988a1..6226bfb 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -1865,7 +1865,7 @@ static void rcu_gp_fqs_loop(void)
> > >   			break;
> > >   		/* If time for quiescent-state forcing, do it. */
> > >   		if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
> > > -		    (gf & RCU_GP_FLAG_FQS)) {
> > > +		    (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
> > >   			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
> > >   					       TPS("fqsstart"));
> > >   			rcu_gp_fqs(first_gp_fqs);
> > > -- 
> > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > > a Linux Foundation Collaborative Project
> > > 
> 
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation