Re: [PATCH v2 tip/core/rcu 07/18] rcu: Add long-term CPU kicking

Paul E. McKenney
2017-01-23T19:34:55Z

On Sat, Jan 21, 2017 at 12:42:55PM -0800, Josh Triplett wrote:
> On Tue, Jan 17, 2017 at 06:53:46PM -0800, Paul E. McKenney wrote:
> > Commit d2db185bfee8 ("rcu: Remove short-term CPU kicking") removed
> > frequent calls to resched_cpu(), which means that the only time
> > resched_cpu() is invoked is after an RCU CPU stall warning. Although
> > this is good from an avoid-IPIs perspective, we should try to break
> > things loose -before- splatting. This commit therefore starts invoking
> > resched_cpu() for each holdout at each force-quiescent-state interval
> > that is more than halfway through the stall-warning interval.
>
> Just realized an issue with this commit message: you're referring to
> what now appears as patch 8 in the past tense as something already done,
> and with a commit ID that probably doesn't work anymore. You need to
> rephrase this to describe how it leads to a *subsequent* change rather
> than fixing something already changed.
>
> With the commit message fixed:
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Good point! It now reads as follows:

------------------------------------------------------------------------
This commit prepares for the removal of short-term CPU kicking (in a
subsequent commit). It does so by starting to invoke resched_cpu()
for each holdout at each force-quiescent-state interval that is more
than halfway through the stall-warning interval.
------------------------------------------------------------------------

Does that work?

Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
>
>
> > ---
> > kernel/rcu/tree.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 83bf054e194e..0e61b62e3f4a 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1225,6 +1225,12 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> > rdp->rsp->gp_start + 2 * jiffies_till_sched_qs) ||
> > ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
> > resched_cpu(rdp->cpu); /* Force CPU into scheduler. */
> > + /*
> > + * If more than halfway to RCU CPU stall-warning time, do
> > + * a resched_cpu() to try to loosen things up a bit.
> > + */
> > + if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2)
> > + resched_cpu(rdp->cpu);
> >
> > return 0;
> > }
> > --
> > 2.5.2
> >
>