Re: [PATCH] rcu: Skip additional checks if rcu_cpu_stall_suppress is set

[Neeraj Upadhyay]

On 08/08/2017 11:32 PM, Paul E. McKenney wrote:
> On Tue, Aug 08, 2017 at 10:50:26PM +0530, Neeraj Upadhyay wrote:
> > If rcu_kick_kthreads is set, and gp is in progress, check_cpu_stall()
> > does checks to figure out whether jiffies is past rsp->jiffies_stall,
> > doing ordered accesses to avoid any false positives for new grace
> > period initialization after a sufficiently large idle period. This
> > extra processing can be skipped if rcu_cpu_stall_suppress is set.
> Just to make sure I understand, the concern is that someone might have
> booted with rcupdate.rcu_cpu_stall_suppress=1 (thus suppressing the RCU
> CPU stall debugging warnings implemented later in check_cpu_stall()),
> but later decided to also boot with rcutree.rcu_kick_kthreads=1 (thus
> enabling kicking kthreads which check for RCU's grace-period kthreads
> not being properly awakened)?
>
> My immediate reaction is that if there is not much point in specifying
> both rcutree.rcu_kick_kthreads=1 and rcupdate.rcu_cpu_stall_suppress=1.
> But is there some use case that I am missing?
>
> 							Thanx, Paul
For boot time configuration, agree, there isn't much point in enabling
both options. In addition to boot time, rcu_cpu_stall_suppress can
be temporarily enabled during some operations like sysrq; but this may
not be a use case worth consideration.
> > Fixes: 8c7c4829a81c ("rcu: Awaken grace-period kthread if too long since FQS")
> > Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > ---
> >   kernel/rcu/tree.c | 7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 51d4c3a..91b7552 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1562,10 +1562,13 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
> >   	unsigned long js;
> >   	struct rcu_node *rnp;
> >
> > -	if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
> > -	    !rcu_gp_in_progress(rsp))
> > +	if (!rcu_gp_in_progress(rsp))
> >   		return;
> >   	rcu_stall_kick_kthreads(rsp);
> > +
> > +	if (rcu_cpu_stall_suppress)
> > +		return;
> > +
> >   	j = jiffies;
> >
> >   	/*
> > --
> > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> > member of the Code Aurora Forum, hosted by The Linux Foundation

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation