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 <neer...@codeaurora.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
