On Wed, Jul 01, 2015 at 12:07:30PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2015 at 02:48:30PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
>
> This seems like a good place to explain why this is a desirable thing,
> no?
Good point.
> Why would you want to limit this?
Because the unconditional wakeup is a two-edges sword. It reduces
the latency of normal RCU grace periods on the one hand, but it makes
rcu_sched consume even more CPU on the other.
Thanx, Paul
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > ---
> > kernel/rcu/tree.c | 15 ++++++++++++---
> > 1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 308b6acb4260..247aa1120c4c 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3505,10 +3505,19 @@ void synchronize_sched_expedited(void)
> > !atomic_read(&rsp->expedited_need_qs));
> >
> > rcu_exp_gp_seq_end(rsp);
> > - mutex_unlock(&rnp->exp_funnel_mutex);
> > smp_mb(); /* ensure subsequent action seen after grace period. */
> > - if (rsp->gp_kthread && rcu_gp_in_progress(rsp))
> > - wake_up(&rsp->gp_wq);
> > + if (rsp->gp_kthread && rcu_gp_in_progress(rsp)) {
> > + static unsigned long nextgp;
> > + static unsigned long nextjiffy;
> > +
> > + if (time_after_eq(jiffies, nextgp) ||
> > + ULONG_CMP_GE(rsp->gpnum, nextgp)) {
> > + nextgp = rsp->gpnum + 4;
> > + nextjiffy = jiffies + 10;
> > + wake_up(&rsp->gp_wq);
> > + }
> > + }
> > + mutex_unlock(&rnp->exp_funnel_mutex);
> >
> > put_online_cpus();
> > }
> > --
> > 1.8.1.5
> >
>
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